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howto:hambasics:sections:test [2021/01/03 11:56] – [Euler Identity and Polar-Cartesian Representations] va7fihowto:hambasics:sections:test [2021/02/13 19:14] (current) – [Cartesian vs Polar] va7fi
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- 
 ====== Optional Math of Waves ====== ====== Optional Math of Waves ======
 This is a brief survey of the math required to analyze waves at the first or second year university level.  If you did well in grade 12 high school math, you'll probably be able to follow this and learn some new and really cool math. This is a brief survey of the math required to analyze waves at the first or second year university level.  If you did well in grade 12 high school math, you'll probably be able to follow this and learn some new and really cool math.
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 \$$ \Box ^2 = -25 \$$  \$$ \Box ^2 = -25 \$$ 
  
-It turns out that there is no //real// number such that when you multiply it by itself you get a negative number, but could we invent an //imaginary// one? +It turns out that there is no //real// number such that when you multiply it by itself you get a negative number.  But could we invent an //imaginary// one?  
 ===== Complex Numbers ===== ===== Complex Numbers =====
 Let's create an //imaginary// number called \$ i \$ such that: Let's create an //imaginary// number called \$ i \$ such that:
- 
  
 <WRAP center box 30em> <WRAP center box 30em>
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 \begin{align*} \begin{align*}
-(1+i)^2 &= 1 + 2i + i^2 \\+(1+i)^2  
 +&= (1+i)\cdot(1+i) \\ 
 +&= 1 + 2i + i^2 \\
 &= 1 + 2i - 1 \\ &= 1 + 2i - 1 \\
 &= 2i &= 2i
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 In a certain way, negative numbers are just as weird as complex numbers: after all, we know what 5 cars look like, but what does −5 cars mean?  And yet, in certain context (like temperature), we have no problem using negative numbers.  Could it be that there are contexts where complex numbers make sense? In a certain way, negative numbers are just as weird as complex numbers: after all, we know what 5 cars look like, but what does −5 cars mean?  And yet, in certain context (like temperature), we have no problem using negative numbers.  Could it be that there are contexts where complex numbers make sense?
 +
 ===== The Complex Plane ===== ===== The Complex Plane =====
 In the same way that we can represent real numbers by a point on the real number line...((Number line picture from [[wp>Number_line |Wikipedia: Number line]])): In the same way that we can represent real numbers by a point on the real number line...((Number line picture from [[wp>Number_line |Wikipedia: Number line]])):
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 ... we can also represent a complex number graphically on a complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part.  For example, \$ (1 + i) \$ would be represented as a point 45° up the horizontal axis and \$ \sqrt{2} \$ away from the origin: ... we can also represent a complex number graphically on a complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part.  For example, \$ (1 + i) \$ would be represented as a point 45° up the horizontal axis and \$ \sqrt{2} \$ away from the origin:
 +
 +<WRAP center round info 80%>
 +You can move the point around to look at other complex numbers on the plane.
 +</WRAP>
  
 {{ggb>/howto/hambasics/sections/polar.ggb}} {{ggb>/howto/hambasics/sections/polar.ggb}}
- 
-You can move the point around to see what the polar representation \$ (r \angle \theta) \$ is. 
  
 To convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, only simple trigonometry and Pythagoras is needed. To convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, only simple trigonometry and Pythagoras is needed.
  
-^ \$$ a + ib \rightarrow r\angle \theta \$$ ^ \$$ r\angle \theta \rightarrow a+ib \$$ |+^ \$$ (a, b) \rightarrow (r\angle \theta\$$ ^ \$$ (r\angle \theta\rightarrow (a, b)\$$ |
 | \$$ r^2 = a^2 + b^2 \$$  |  \$$ a = r\cos\theta \$$  | | \$$ r^2 = a^2 + b^2 \$$  |  \$$ a = r\cos\theta \$$  |
 | \$$ \tan \theta = \dfrac{b}{a} \$$ | \$$ b = r\sin\theta \$$ | | \$$ \tan \theta = \dfrac{b}{a} \$$ | \$$ b = r\sin\theta \$$ |
 +
 +Note that very often, we use radians instead of degrees for the angle.  There are a total 360° or 2π radians in a circle.  While most people are used to degrees, a radian is actually much easier to picture:
 +  * Imagine a circle.
 +  * Now imagine the length from the centre to the circle (along the "radius").
 +  * Take that length and lay it down on the perimeter of the circle.
 +  * The angle that this length covers is 1 radian (because of the length of the radius on the circle).
 +  * That's why a circle has 2π radians (because the circumference is 2πr) 
  
 ==== Roots ==== ==== Roots ====
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 Without using the graph above, what do you expect the solution(s) to \$ z^3 = 8 \$ will be?  That is, what number(s), when multiplied by itself three times gives 8? Without using the graph above, what do you expect the solution(s) to \$ z^3 = 8 \$ will be?  That is, what number(s), when multiplied by itself three times gives 8?
  
-Now move \$ w = 8 \$ and \$n = 3 \$ to have a look at the solutions.  +Now move \$ w = 8 \$ and \$n = 3 \$ to have a look at the solutions graphically, you might be surprised by what you find.
  
 <hidden> <hidden>
 {{ complexroots.png?600 }} {{ complexroots.png?600 }}
-So \$z = 2\$ was to be expected since \$ 2^3 = 8 \$ but it looks like there'two more solutions. \\ \\+So \$z = 2\$ was to be expected since \$ 2^3 = 8 \$ but it looks like there are two more solutions. \\ \\
  
 To find them, we first notice that the three solutions are spread out evenly around the circle, that is they are 120° apart. To find them, we first notice that the three solutions are spread out evenly around the circle, that is they are 120° apart.
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 </hidden> </hidden>
 \\ \\
- 
  
 ===== The Euler Identity ===== ===== The Euler Identity =====
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 </WRAP> </WRAP>
  
-Let's use two different ways to verify that this mysterious identity works.+Let's use two different ways to verify that this mysterious identity is true.
  
 ==== The Derivatives ==== ==== The Derivatives ====
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 ===== Euler Identity and Polar-Cartesian Representations ===== ===== Euler Identity and Polar-Cartesian Representations =====
  
-In the previous section, we saw that a complex number \$z = a + ib \$ could be represented as a point \$(a, b)\$ on the complex plane, which could also be viewed in polar coordinates as \$r\angle \theta \$.  We saw that to convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, only simple trigonometry and Pythagoras is needed:+In the previous section, we saw that a complex number \$z = a + ib \$ could be represented as a point \$(a, b)\$ on the complex plane, which could also be viewed in polar coordinates as (\$r\angle \theta\$.  We saw that to convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, only simple trigonometry and Pythagoras is needed:
  
-^ \$$ a + ib \rightarrow r\angle \theta \$$ ^ \$$ r\angle \theta \rightarrow a+ib \$$ |+^ \$$ (a,b) \rightarrow (r\angle \theta\$$ ^ \$$ (r\angle \theta\rightarrow (a,b)\$$ |
 | \$$ r^2 = a^2 + b^2 \$$  |  \$$ a = r\cos\theta \$$  | | \$$ r^2 = a^2 + b^2 \$$  |  \$$ a = r\cos\theta \$$  |
 | \$$ \tan \theta = \dfrac{b}{a} \$$ | \$$ b = r\sin\theta \$$ | | \$$ \tan \theta = \dfrac{b}{a} \$$ | \$$ b = r\sin\theta \$$ |
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 \end{align*} \end{align*}
  
-This offers another interpretation of the Euler identity:+This offers another interpretation of the Euler identity as the algebraic conversion between Cartesian and Polar coordinates:
  
-^           ^Cartesian        ^Polar +^            Cartesian       Polar  |
-^ Algebraic |\$$z = a + ib\$$ |\$$ z = re^{i\theta} \$$ |+
 ^ Graphical |\$$(a, b) \$$    |\$$ (r\angle \theta) \$$  | ^ Graphical |\$$(a, b) \$$    |\$$ (r\angle \theta) \$$  |
 +^ Algebraic |\$$z = a + ib\$$ |\$$ z = re^{i\theta} \$$ |
  
 +This now allows us to simplify a lot of difficult mathematics.  For example let's look at the root problem \$z^3 = 8 \$ again.  Since the number 8 on the complex plane is the point \$(8,0)\$, in polar coordinates, it can be any of the following: \$8\angle 0, 8\angle 2\pi, 8\angle 4\pi, \cdots \$  This is because we can go around the circle as many times as we want and return to the same point.  Since we expect three roots, let's use the first three polar representations of 8:
 +
 +\$$
 +z^3 = 8 = \left\{
 +\begin{array}{c}
 +8e^{0 i} \\
 +8e^{2\pi i} \\
 +8e^{4\pi i}\\
 +\vdots 
 +\end{array}
 +\right.
 +\$$
 +
 +\$$
 +\Rightarrow z = \left\{
 +\begin{array}{lcl}
 +\left(8e^{0 i}\right)^{\frac{1}{3}} &=& 8^{\frac{1}{3}} e^{\frac{0}{3}} = 2\\
 +\left(8e^{2\pi i}\right)^{\frac{1}{3}} &=& 8^{\frac{1}{3}} e^{\frac{2\pi}{3}i} = 2 \left(\cos \frac{2\pi}{3} + i \sin \frac{2\pi}{3}\right) = 2 \left(-\frac{1}{2} + i \frac{\sqrt{3}}{2}\right) = -1 + i\sqrt{3} \\
 +\left(8e^{4\pi i}\right)^{\frac{1}{3}} &=& 8^{\frac{1}{3}} e^{\frac{4\pi}{3}i} = 2 \left(\cos \frac{4\pi}{3} + i \sin \frac{4\pi}{3}\right) = 2 \left(-\frac{1}{2} - i \frac{\sqrt{3}}{2}\right) = -1 - i\sqrt{3} \\
 +&\vdots& 
 +\end{array}
 +\right.
 +\$$
 +
 +If we had used more than 3 numbers, the roots would have started repeating.  If we had used less than 3, we would have missed some answers in the same way that there are two answers to \$z^2 = 9 \$ (namely \$z = \pm 3 \$).
 +
 +===== Cartesian vs Polar =====
 +Which is the best representation: Cartesian, \$z = a + i b \$, or Polar, \$z = re^{i\theta}\$.  As you might expect, it depends on what you're trying to do...  For example, let's take:
 +
 +\$$z_1 = 1 + i = \sqrt{2}e^\left(i\frac{\pi}{4}\right) \quad \text{and} \quad z_2 = -1 + i = \sqrt{2}e^\left(i\frac{3\pi}{4}\right) \$$
 +
 +Imagine having to add, subtract, multiply, or divide these together.  Or raise them to a power, or take a root of them.  Which of the two representations do you think would be easiest to use for each operation?
 +
 +<hidden>
 +|< 100% - 50% 50% >|
 +^Operation                Cartesian  ^  Polar  ^
 +^Addition        Easiest: \$$ z_1 + z_2 = (1 + i) + (-1 + i) = 2i \$$  |  I don't know how to do it.  \$$ z_1 \pm z_2 = \sqrt{2}e^\left(i\frac{\pi}{4}\right) \pm \sqrt{2}e^\left(i\frac{3\pi}{4}\right) = ??  \$$  |
 +^Subtraction    |  Easiest: \$$ z_1 - z_2 = (1 + i) - (-1 + i) = 2 \$$    :::  |
 +^Multiplication |  Moderate: \$$ z_1 \cdot z_2 = (1 + i) \cdot (-1 + i) \$$ \$$ = (1)(-1) + (1)(i) + (i)(-1) + (i)(i) \$$  \$$ = -1 + i - i -1 = -2 \$$  |  Easiest:  \$$ z_1 \cdot z_2 = \sqrt{2}e^\left(i\frac{\pi}{4}\right) \cdot \sqrt{2}e^\left(i\frac{3\pi}{4}\right)  \$$  \$$ = \sqrt{2}\cdot \sqrt{2} e^\left(i\frac{\pi}{4} + i\frac{3\pi}{4}\right) \$$  \$$ = 2 e^{i\pi} = -2 \$$  |
 +^Division |  Tedious: \$$ \frac{z_1}{z_2} = \frac{(1 + i)}{(-1 + i)} = \frac{(1 + i)(-1 - i)}{(-1 + i)(-1 - i)} \$$  \$$ = \cdots = \frac{-2i}{2} = -i  \$$  |  Easiest:  \$$ \frac{z_1}{z_2} = \frac{\sqrt{2}e^\left(i\frac{\pi}{4}\right)}{\sqrt{2}e^\left(i\frac{3\pi}{4}\right)}  = e^\left(i\frac{\pi}{4} - i\frac{3\pi}{4}\right) \$$  \$$ = e^\left(-i\frac{\pi}{2}\right) = -i \$$  |
 +^Exponentiation |  The bigger the exponent, the more tedious: \$$z_1^{100} = (1 + i)^{100} = \cdots \$$  |  Easy no matter how big the exponent:  \$$z_1^{100} =  \left(\sqrt{2}e^\left(i\frac{\pi}{4}\right)\right)^{100} \$$  \$$ =  2^{50}e^{25\pi i} = 2^{50}e^{12(2\pi i)} e^{\pi i} \$$ \$$ = 2^{50}(1)(-1) = -2^{50}\$$  |
 +^Roots          |  I don't know if it's even possible: \$$ \sqrt[3]{z_2} = \sqrt[3]{-1 + i}\$$ \$$ (-1 + i)^\left(\frac{1}{3}\right) = ??  \$$  |  As easy as exponentiation:  \$$ \sqrt[3]{z_2} = \left(\sqrt{2}e^\left(i\frac{3\pi}{4}\right) \right)^\left(\frac{1}{3} \right) \$$ \$$ = \sqrt[6]{2}e^\left(i\frac{\pi}{4}\right) = \frac{1 + i}{\sqrt[3]{2}}  \$$  |
 +</hidden>
 +\\
  
 +The lesson here is that since the polar representation uses exponents, and exponents turn multiplication into addition((\$ x^a \cdot x^b = x^{a+b} \$)), the polar representation is easiest for multiplication, division, exponentiation, and roots.  It's essentially why the [[howto/hambasics/sections/mathbasics#the_decibel |dB scale]] is so useful.  But addition and subtraction is intrinsically easier in Cartesian coordinates.
 ===== Important Algebraic Results ===== ===== Important Algebraic Results =====
   * Use the Euler identity to get the following two useful results:   * Use the Euler identity to get the following two useful results:
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 \\ \\
  
-===== Équations différentiels =====+This last result is the basis behind why [[howto/hambasics/sections/wavemodulationmath |modulating the amplitude]] of a carrier produces side bands.
  
-Dans la physique des ondes, on aura bientôt à trouver la solutions d'une équation différentiel de ce type: 
-\$$ a \ddot{x}(t) + b \dot{x}(t) + c x(t) = 0 \$$  
  
-Dans nos applications, les paramètres \$ a, b, c \$ serons tous des quantités réels et positive. Même sans avoir étudier le sujet d'équation différentiels en profondeurs, on peut imaginer qu'une solution possible serait \$ x(t)e^{rt} \$ puisque la dérivé d'une fonction exponentiel est elle même une fonction exponentiel, ce qui est encourageant... L'étape suivante est donc d'essayer cette fonction dans l'équation différentiel, pour trouver les valeurs de \$ r \$ qui fonctionnent. Donc:+===== Differential Equations =====
  
 +In the physics of wave, we often have to find solutions to the following type of differential equations:
 +<WRAP center box 30em>
 +\$$ a \ddot{x}(t) + b \dot{x}(t) + c x(t) = 0 \$$
 +</WRAP>
  
-\$ \begin{align*}+<WRAP center round info 80%> 
 +  * A differential equation is an equation that relates a function to its derivatives in some ways and the question is: given some information about the system, what's the function (or family of functions) that satisfy the differential equation. 
 + 
 +  * In physics we often use a dot above the function to indicate a derivative with respect to time, where as in math, we'll often use an apostrophe.  Physicists don't like the apostrophe too much because they sometimes use it to denote a different coordinate system.  So don't let the notation confuse you: 
 +\$$ \dot{x}(t) = x'(t) = \frac{dx}{dt} \quad \text{and} \quad \ddot{x}(t) = x''(t) = \frac{d^2x}{dt^2} \$$ 
 +</WRAP> 
 + 
 + 
 +In our applications, the parameters  \$ a, b, c \$ are all real and positive quantities.  Even without having studied different equations in any depth, we can imagine that a possible solution to the above differential equation would be: \$ x(t)= e^{rt} \$  since the derivative of an exponential function is itself an exponential function, which is encouraging. 
 + 
 +The next step is to try this ``test function'' in the differential equation and see if we can find the values of \$ r \$ that make it work.  First we'll need derivatives of the test function: 
 + 
 +\begin{align*}
 & x (t) = e^{rt} \\  & x (t) = e^{rt} \\ 
 \Rightarrow \qquad & \dot{x}(t) = r e^{rt} \\  \Rightarrow \qquad & \dot{x}(t) = r e^{rt} \\ 
 \Rightarrow \qquad & \ddot{x}(t) = r^2 e^{rt}  \Rightarrow \qquad & \ddot{x}(t) = r^2 e^{rt} 
-\end{align*} \$+\end{align*}
  
- et notre équation différentiel devient+When we put these into the differential equation, we get:
  
-\$ \begin{align*}+\begin{align*}
 & a \ddot{x}(t) + b \dot{x}(t) + c x(t) = 0 \\  & a \ddot{x}(t) + b \dot{x}(t) + c x(t) = 0 \\ 
 \Rightarrow \qquad & a (r^2 e^{rt}) + b (r e^{rt}) + c (e^{rt}) = 0 \\  \Rightarrow \qquad & a (r^2 e^{rt}) + b (r e^{rt}) + c (e^{rt}) = 0 \\ 
 \Rightarrow \qquad & e^{rt} (a r^2 + b r + c ) = 0 \\  \Rightarrow \qquad & e^{rt} (a r^2 + b r + c ) = 0 \\ 
-\Rightarrow \qquad & a r^2 + b r + c = 0  +\Rightarrow \qquad & a r^2 + b r + c = 0 \\ 
-\end{align*} \$+\Rightarrow \qquad &r = - \dfrac{b}{2a} \pm \dfrac{\sqrt{b^2 - 4ac}}{2a} 
 +\end{align*}
  
-\$ \Rightarrow \qquad r = - \dfrac{b}{2a} \pm \dfrac{\sqrt{b^2 - 4ac}}{2a} \$+So what does that result mean?  Remember, what we're looking for is the function \$x(t)\$ that satisfies the differential equation \$ a \ddot{x}(t) + b \dot{x}(t) + c x(t) = 0 \$
  
-Puisque \$ r \$ contient une racine carré, il peut être réel, ou complexe. Pour simplifier la notation, disons que: \$ \alpha = \dfrac{b}{2a} \qquad \text{et} \qquad \beta = \dfrac{\sqrt{|{b^2 - 4ac}|}}{2a} \$ 
  
-Nous pourrons donc dire que: \$ \begin{equation*} r = \left\{ \begin{array}{rl} -\alpha \pm \beta & \text{si } b^2 - 4ac > 0,\\  
- -\alpha \pm i \beta & \text{si } b^2 - 4ac < 0, \end{array} \right. \end{equation*} \$ 
  
-Examinons les deux cas en plus de détails.+What we've go so far says that our test function \$x(t) = e^{rt}\$ will satisfy the differential equation if \$r\$ is given by above equation.  There is still a lot to unpack however.  For example, since \$r\$ contains a square root, it could be real or complex depending on the values of \$a, b,\$ and \$c\$.  And as we saw above, if \$r\$ is real, then \$x(t)\$ will be a real exponential function.  But if \$r\$ is complex, then we can expect \$x(t)\$ to be some sort of sinusoidal function (recall the Euler Identity).
  
-=== Cas 1=== +To simplify the notation, let's define \$\alpha\$ and \$\beta\$ as
-Quand \$ b^2 - 4ac > 0 \$ , \r \$ est réel et la solutions général sera: +\$$ \alpha = \dfrac{b}{2a} \qquad \text{and} \qquad \beta = \dfrac{\sqrt{|{b^2 - 4ac}|}}{2a} \$$
  
-\$ \begin{align*}+Notice how the absolute value under the square root ensures that \$\beta\$ is always real. 
 + 
 +\$r\$ then:  
 + 
 +\$$ r = \left\{ \begin{array}{ll} -\alpha \pm \beta & \text{if } b^2 - 4ac > 0,\\  
 + -\alpha \pm i \beta & \text{if } b^2 - 4ac < 0, \end{array} \right. \$$ 
 + 
 +Let's examine both of these cases in more detail. 
 + 
 +==== Case 1: Over Damped Oscillation ==== 
 +When \$ b^2 - 4ac > 0 \$ , \$ r \$ is real and the general solution is:  
 + 
 +\begin{align*}
 x(t) &= A_1 e^{r_1 t} + A_2 e^{r_2 t} \\  x(t) &= A_1 e^{r_1 t} + A_2 e^{r_2 t} \\ 
 &= A_1 e^{( -\alpha + \beta) t} + A_2 e^{( -\alpha - \beta) t} \\  &= A_1 e^{( -\alpha + \beta) t} + A_2 e^{( -\alpha - \beta) t} \\ 
 &= A_1 e^{-\alpha t} e^{\beta t} + A_2 e^{-\alpha t} e^{-\beta t}  &= A_1 e^{-\alpha t} e^{\beta t} + A_2 e^{-\alpha t} e^{-\beta t} 
-\end{align*} \$+\end{align*}
  
 \$$ x(t) = e^{-\alpha t} ( A_1 e^{\beta t} + A_2 e^{-\beta t} ) \$$ \$$ x(t) = e^{-\alpha t} ( A_1 e^{\beta t} + A_2 e^{-\beta t} ) \$$
  
-C'est normal d'avoir deux constantes d'intégrations puisque notre équation différentiel une dérivé du second degréPour trouver ces constantesça nous prendrait des valeurs initiales.+It'normal to have two constants of integration since our differential equation has a second degree derivative in it To find these constantswe'd need to know more about the system's initial conditions.
  
-=== Cas 2: === +==== Case 2: Under Damped ==== 
-Quand \$ b^2 - 4ac < 0 \$ , \$ r \$ est complexe et nous utiliserons la formule de Euler pour simplifier notre solutions.+When \$ b^2 - 4ac < 0 \$ , \$ r \$ is complex and we'll be using the Euler identity to simplify our solutions
    
-\$ \begin{align*}+\begin{align*}
 x(t) &= A_1 e^{r_1 t} + A_2 e^{r_2 t} \\  x(t) &= A_1 e^{r_1 t} + A_2 e^{r_2 t} \\ 
 &= A_1 e^{( -\alpha + i \beta) t} + A_2 e^{( -\alpha - i \beta) t} \\  &= A_1 e^{( -\alpha + i \beta) t} + A_2 e^{( -\alpha - i \beta) t} \\ 
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 &= e^{-\alpha t} \Big( (A_1 + A_2) \cos(\beta t) + i (A_1 - A_2) \sin(\beta t) \Big) \\  &= e^{-\alpha t} \Big( (A_1 + A_2) \cos(\beta t) + i (A_1 - A_2) \sin(\beta t) \Big) \\ 
 &= e^{-\alpha t} \Big( a_1 \cos(\beta t) + a_2 \sin(\beta t) \Big) \\  &= e^{-\alpha t} \Big( a_1 \cos(\beta t) + a_2 \sin(\beta t) \Big) \\ 
-&= e^{-\alpha t} \Big( A \sin \phi \cos(\beta t) + A \cos \phi \sin(\beta t) \Big)  +&= e^{-\alpha t} \Big( A \sin \phi \cos(\beta t) + A \cos \phi \sin(\beta t) \Big)  \\ 
-\end{align*} \$+&= Ae^{-\alpha t} \Big(\sin \phi \cos(\beta t) + \cos \phi \sin(\beta t) \Big)  \\ 
 +\end{align*}
  
-Dans les deux dernière lignenous avons ré-écrit les constantes d'intégration:+In the last three lineswe've redefined the constants of integration a few times so that:
  
-\$ \begin{align*}+\begin{align*}
 a_1 &= A_1 + A_2 & , a_2 &= i(A_1 - A_2) \\  a_1 &= A_1 + A_2 & , a_2 &= i(A_1 - A_2) \\ 
 a_1 &= A \sin \phi & , a_2 &= A \cos \phi  a_1 &= A \sin \phi & , a_2 &= A \cos \phi 
-\end{align*} \$ +\end{align*}
- +
-Pour finalement pouvoir utiliser une identité trigonométrique:+
  
 +And we finally use one of the trig identities we proved earlier to write the solution as:
 \$$ x(t) = A e^{-\alpha t} \sin(\beta t + \phi) \$$ \$$ x(t) = A e^{-\alpha t} \sin(\beta t + \phi) \$$
  
-=== Exemple ===+==== Case 3: Critically Damped ==== 
 +When \$ b^2 - 4ac = 0 \$ , \$ r = -\frac{b}{2a} \$ is real and negative but our test solution is under determined.  We'll instead propose a solution of the following type and test that it works: 
 +\begin{align*} 
 + && x(t) &= e^{rt}(A + Bt) \\ 
 +\Rightarrow && \dot x(t) &= re^{rt}(A + Bt) + Be^{rt} \\ 
 +            &&           &= e^{rt}\big(r(A + Bt) + B)\big) \\ 
 +            &&           &= e^{rt}(rA + B + Brt) \\ 
 +\Rightarrow &&\ddot x(t) &= re^{rt}(rA + B + Brt) + e^{rt}Br \\ 
 +            &&           &= e^{rt}\big(r(rA + B + Brt) + Br\big) \\ 
 +            &&           &= e^{rt}(Ar^2 + 2Br + Br^2t) \\ 
 +\end{align*} 
 + 
 + 
 + 
 + 
 +==== Exemple ====
 Nous avons donc deux types de solutions complètement différents qui dépendent de trois paramètres \$ a, b, c \$. Pour voir comment ces paramètres affectent le graphique, imaginons qu'une de nos conditions initiales est \$ \phi = \frac{\pi}{2} \$ . Ça veut dire que: Nous avons donc deux types de solutions complètement différents qui dépendent de trois paramètres \$ a, b, c \$. Pour voir comment ces paramètres affectent le graphique, imaginons qu'une de nos conditions initiales est \$ \phi = \frac{\pi}{2} \$ . Ça veut dire que:
  
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howto/hambasics/sections/test.1609703809.txt.gz · Last modified: 2021/01/03 11:56 by va7fi