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Topics - xuanzhong

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1
Quiz 3 / quiz 3 lec5101 - 5b
« on: October 08, 2020, 10:16:37 PM »
Compute line integral: $\int_{r} Re(z) \,dz$, where r is line segmemt from 1 to i
$$
f(z) = Re(z)
$$
$$
r(t) = (1-t)z_0 + tz_1 = (1-t)*1 + ti = 1-t+it , 0<= t <= 1
$$
$$
r^{'}(t) = i-1
$$
$$
f(r(t)) = Re(r(t)) = 1-t
$$
$$
\int_{r} f(z) \,dz = \int_{0}^{1}f(r(t))r^{'}(t) \,dt
$$
$$
=\int_{0}^{1}(1-t)(i-1) \,dt
$$
$$
=(i-1)*(t-\tfrac{1}{2} t^2)\Big|_0^1
$$
$$
=(i-1)(1-\tfrac{1}{2}-0)
$$
$$
=\tfrac{1}{2}(i-1)
$$

2
Quiz 1 / quiz 1 5101
« on: September 25, 2020, 07:44:43 PM »
Find the locus of points for:
$$
Re(z^{2}) = 4
$$
$$
Let z=x+iy
$$
$$
Re((x+iy)^{2}) = 4
$$
$$
Re(x^{2}+2ixy-y^{2}) = 4
$$
$$
x^{2} - y^{2} = 4
$$
$$
\frac{x^{2}}{4} - \frac{y^{2}}{4} = 1
$$
Therefore, the locus of z is a hyperbolic curve with $\frac{x^{2}}{4} - \frac{y^{2}}{4} = 1$

3
Quiz-5 / Lec5101 quiz5
« on: November 01, 2019, 01:47:05 PM »
Find the general solution of the given differential equation.
$$
y^{\prime\prime}+4y=3csc(2t), 0<t<pi/2
$$
For homogeneous equation: $r^2+4=0$
we get:
$$
r_{1}=2i, r_{2}=-2i
$$
$$
y_{c}(t)=c_{1}cos2t+c_{2}sin2t
$$

For non-homogeneous equation:
$$
\begin{equation}
    W[y_{1},y_{2}](t) = \begin{vmatrix}
      cos2t       & sin2t \\
      -2sin2t       & 2cos2t \\
    \end{vmatrix} = 2
  \end{equation}
$$

Therefore,
$$
u_{1}(t)=-(\int\frac{sin2t*3csc2t}{2}dt)
$$
$$
=-(\int\frac{3}{2}dt)
$$
$$
=-\frac{3}{2}t
$$
$$
u_{2}(t)=-(\int\frac{cos2t*3csc2t}{2}dt)
$$
$$
=\frac{3}{2}(\int\frac{cos2t}{sin2t}dt)
$$
$$
=\frac{3}{2}(\int{cot2t}dt)
$$
$$
=\frac{3}{4}ln|sin2t|
$$

Hence, the particular solution is $y_{p}(t)=u_{1}(t)y_{1}(t)+u_{2}(t)y_{2}(t)$
$$
y_{p}(t)=cos2t\cdot(-\frac{3}{2}t)+sin2t\cdot(\frac{3}{4}ln|sin2t|)
$$
$$
=\frac{3}{4}sin2tln|sin2t|-\frac{3}{2}tcos2t
$$

Therefore, the general solution is:
$$
y(t)=y_{c}(t)+y_{p}(t)
$$
$$
=c_{1}cos2t+c_{2}sin2t+\frac{3}{4}sin2tln|sin2t|-\frac{3}{2}tcos2t
$$

4
Quiz-3 / TUT0601 quiz3
« on: October 11, 2019, 06:29:27 PM »
Find the Wronskian of two solutions of the given differential equation without solving the equation.
$$
cos(t)y^{\prime\prime}+sin(t)y^\prime-ty=0
$$

First, we divide both sides of the equation by cos(t):
$$
y^{\prime\prime}+\frac{sin(t)}{cos(t)}y^\prime-\frac{1}{cos(t)}y=0
$$

Now the given second-order differential equation has the form:
$$
L[y]= y^{\prime\prime}+p(t)y^\prime-\ q(t)y=0
$$

Then by Abel’s Theorem: the Wronskian W[y1,y2](t) is given by
$$
W[y_1,y_2\ ](t)=cexp(-\int p(t)dt)
$$
$$
=cexp(-\int\frac{sin(t)}/{cos(t)}dt〗)
$$
$$
=cexp(\int\frac{1}{cos(t)}\ d(cos(t)〗)
$$
$$
=ce^{ln|cos(t)|}\
$$
$$
=ccos(t)
$$

5
Quiz-2 / TUT0601 quiz 2
« on: October 04, 2019, 07:25:01 PM »
Show that the given equation is not exact but becomes exact when multiplied by the given integrating factor. Then solve the equation.
$$
\frac{x^2}{y^3}+x(1+y^2\ )\frac{dy}{dx} =0,\mu(x,y)=\frac{1}{xy^3}
$$
First, define $M(x,y)=\ x^2\ y^3$,$N(x,y)=x(1+y^2)$
$$
M_y=\partial y/\partial x\ (x^2\ y^3\ )=3x^2\ y^2
$$
$$
N_y=\partial y/\partial x[x(1+y^2\ )]=1+y^2
$$

Since $M_y\neq N_y$, this implies the given equation is not exact.
Now, multiplying both sides by the integrating factor $\mu(x,y)$, that is
$$
\frac{1}{xy^3}x^2y^3+\frac{1}{xy^3}x(1+y^2\ )\frac{dy}{dx}=x+(\frac{1}{y^3}+\frac{1}{y}) \frac{dy}{dx}=0
$$

Define $M^\prime\ (x,y)=\ x$,$N^\prime\ (x,y)=\frac{1}{y^3}+\frac{1}{y}$
$$
M_y=\partial y/\partial x\ (x)=0
$$
$$
N_y=\partial y/\partial x[\frac{1}{y^3}+\frac{1}{y}]=0
$$

Since $M_y$=$N_y$, this implies the given equation is exact.
Thus, we know there exists a function $\phi(x,y)$ such that $\partial\phi/\partial x=\ M^\prime\ (x,y)=\ x$
Then $\phi(x,y)=\int M^\prime\ dx=\frac{1}{2} x^2+h(y)$
Take derivative on both sides with respect to y we get
$$
\partial\phi/\partial y=h^\prime\ (y)=N^\prime\ (x,y)=\frac{1}{y^3}+\frac{1}{y}
$$

Integrating with respect to y we have
$$
h(y)=\frac{-1}{2}\frac{1}{y^2}+ln|y|+C
$$

Hence, $\phi(x,y)=\frac{1}{2} x^2-\frac{1}{2} \frac{1}{y^2}+ln|y|=C$ is the general solution to the given DE.
Besides, notice that the constant function y(x)=0\ \forall x is also a solution to the given DE.



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