MAT244--2019F > Term Test 1
Problem 2 (afternoon)
huoyanro:
--- Quote from: Lan Cheng on October 23, 2019, 07:04:14 AM ---
a) divide each side by $x(2x+1)$:
$y”+\frac{2x+2}{x(2x+1)}y’-\frac{2}{x(2x+1)}y=0.$
$p(x)=\frac{2x+2}{x(2x+1)},W=ce^{-\int p(x)dx}.$
let $p(x)=\frac{A}{x}+\frac{B}{2x+1}.$
$p(x)=\frac{(2A+B)x+A}{x(2x+1)}, A=2,B=-2.$
$p(x)=\frac{2}{x}-\frac{2}{2x+1}.$
$\int-p(x)dx=\int\frac{2}{2x+1}-\frac{2}{x}dx=ln(2x+1)-2ln(x).$
$W=ce^{ln(2x+1)-2ln(x)}=ce^{ln(2x+1)}e^{ln(x^{-2})}=c(2x+1)(\frac{1}{x^{2}})=c(\frac{2}{x}+\frac{1}{x^{2}})$
b) let $c=1,W=\frac{2}{x}+\frac{1}{x^{2}}=\begin{array}{cc}
x+1 & y_{2}\\
1 & y_{2}'
\end{array}=(x+1)y_{2}'-y_{2}.$
divide each side by $(x+1): y_{2}'-\frac{1}{x+1}y_{2}=\frac{2}{x(x+1)}+\frac{1}{x^{2}(x+1)}.$
$\mu=e^{\int p_{2}(x)dx}=e^{\int-\frac{1}{x+1}dx}=e^{-ln(x+1)}=\frac{1}{x+1}.$
multiply each side by $\mu:\frac{1}{x+1}y_{2}'-\frac{1}{(x+1)^{2}}y_{2}=\frac{2}{x(x+1)^{2}}+\frac{1}{x^{2}(x+1)^{2}}.$
$\frac{1}{x+1}y_{2}=-\frac{1}{x(x+1)},y_{2}=-\frac{1}{x}.$
Therefore, $y_{1}=x+1,y_{2}=-\frac{1}{x}.$
c)$y(-1)=1,y'(-1)=0.$
$y(x)=C_{1}y_{1}+C_{2}y_{2}=C_{1}(x+1)+C_{2}(-\frac{1}{x}).$
$y'(x)=C_{1}+C_{2}x^{-2}.$
$\begin{cases}
C_{1}=1 & C_{2}=-1\end{cases}.$
Therefore, the general solution is $y(x)=x+1+\frac{1}{x}.$
--- End quote ---
question c is wrong, C2 should be 1
because when x=-1,y=1, $y(x)=C_{1}y_{1}+C_{2}y_{2}=C_{1}(x+1)+C_{2}(-\frac{1}{x}).$
C_{2}=1\end{cases}.$
Lan Cheng:
--- Quote from: Hongling Liu on October 23, 2019, 06:43:41 AM ---2:
(2x+1)xy’’ + (2x+2)y’ -2y = 0
Solution:
a):
y’’ + [(2x+2)/(2x+1)x]y’ -[2/(2x+1)x]y= 0
W = C•e^∫-[(2x+2)/(2x+1)x]dx
W = C•(2x+1/x^2) let C = 0
W = (2x+1/x^2)
--- End quote ---
Changhao Jiang:
(a)
Rewrite the equation:$y''+\frac{2x+2}{x(2x+1)}y'+\frac{2}{x(2x+1)}y=0$
$p(x)=\frac{2x+2}{x(2x+1)}, W=e^{-\int p(x) dx}$
then, by Abel's theorem, $W(y_1,y_2)(x)=ce^{- \int \frac{2x+2}{x(2x+1)}}dx$
We can first calculate the integral $\int \frac{2x+2}{x(2x+1)} dx$
let $\frac{A}{x}+\frac{B}{2x+1}=\frac{2x+2}{x(2x+1)},
then \frac{(2x+1)A+Bx}{x(2x+1)}=\frac{2x+2}{x(2x+1)}$
then we get A=2, B=-2, so the integral $ \int \frac{2x+2}{x(2x+1)} dx = \int 2(\frac{1}{x}-\frac{1}{2x+1}) dx = 2(lnx-\ln(2x+1))=\ln \frac{x^2}{2x+1}$
therefore, $W=ce^{-\ln \frac{x^2}{2x+1}}=c\frac{2x+1}{x^2}$
let c=1, $W=\frac{2x+1}{x^2}$
(b)
$y_1=x+1, y_1'=1, y_1''=0$, put $y_1, y_1'$ and $y_1''$ into the equation, we can get $(2x+1)x \cdot 0+(2x+2) \cdot 1+2(x+1)=0$; therefore; $y_1$ is the solution for ODE
Let another linearly independent solution be $y_2$
Since we know $W=y_1y_2'-y_2y_1'=(x+1)y_2'-y_2$
then we need to solve the equation $(x+1)y_2'-y_2=\frac{2x+1}{x^2}$
Rewrite the equation: $y_2'-\frac{1}{x+1}y_2=\frac{2x+1}{x^2(x+1)}$
$\mu(x)=e^{\int -\frac{1}{x+1} dx}=\frac{1}{x+1}$, multiply on both sides with $\mu(x)$
We can get $\frac{1}{x+1}y_2'-\frac{1}{(x+1)^2}y_2=\frac{2x+1}{x^2(x+1)^2}$
Integrate on both sides, we can get $\frac{1}{1+x}y_2 = -\frac{1}{x(x+1)}+c$
Then $y_2 = -\frac{1}{x}+c(x+1)$, let c=1, $y_2=-\frac{1}{x}+x+1$
(c)
the general solution $y=c_1(x+1)+c_2(-\frac{1}{x}+x+1)$
since y(-1)=1, we can get $c_2=1$
$y'=c_1+c_2+\frac{c_2}{x^2}$, and y'(-1)=0, so $c_1=-2$
Therefore, $y=-(\frac{1}{x}+x+1)$
Xuefen luo:
a)Dividing both sides by $(2x+1)x$, we have:
$y''+\frac{(2x+2)}{(2x+1)x}y'-\frac{2}{(2x+1)x}y=0$
Then, $w=ce^{-\int \frac{(2x+2)}{(2x+1)x} dx} = ce^{-\int \frac{-2}{2x+1} + \frac{2}{x} dx} = ce^{ln|2x+1|-ln|x^2|}=c((2x+1)x^{-2})$
Let $c=1, w=(2x+1)x^{-2}$.
b) $w=(x+1)y_2'-y_2=(2x+1)x^{-2}$
Then, $y_2'-\frac{1}{(x+1)}y_2=\frac{2x+1}{x^2(x+1)}$
$\mu =e^{-\int \frac{1}{x+1} dx}=e^{ln|x+1|}=\frac{1}{x+1}$
Multiplying $\frac{1}{x+1}$ to both sides of $y_2'-\frac{1}{x+1}y_2=\frac{2x+1}{x^2(x+1)}$, we have:
$\frac{1}{x+1}y_2'-\frac{1}{(x+1)^2}y_2=\frac{2x+1}{x^2(x+1)^2}$
Then, $\frac{1}{x+1}y_2=\int \frac{2x+1}{x^2(x+1)^2} dx$
$\frac{1}{x+1}y_2=\int \frac{1}{x^2}-\frac{1}{(x+1)^2} dx$
$\frac{1}{x+1}y_2=-\frac{1}{x}+\frac{1}{(x+1)^2}$
$y_2=-\frac{x+1}{x}+1$
Hence, we get $y=c_1(x+1)+c_2(-\frac{x+1}{x}+1)$
c)We have $y=c_1(x+1)+c_2(-\frac{x+1}{x}+1)$ and $y'=c_1+\frac{1}{x^2} c_2$
Plug in $y(-1)=1,y'(-1)=0$, we get $y(-1)=c_2=1$ and $y'(-1)=c_1+c_2=0$.
Since $c_2=1,c_1=-1$.
Therefore, $y=-(x+1)-\frac{x+1}{x}+1=-x-\frac{1}{x}+1$
Ranran Wang:
\textbf{Q2. }$(2 x+1) x y^{\prime \prime}+(2 x+2) y^{\prime}-2 y=0$. Fnd $w\left(y_{1}, y_{2}\right)$. Check $y_{1}(x)=x+1$ is $a$ solution and fnd another linearly independent solution. Write general solution, and find solutions such fhat $y(-1)=1, \quad y^{\prime}(-1)=0$.
\textbf{Ans:} $y^{\prime \prime}+\frac{2 x+2}{(2 x+1) x} y^{\prime}-\frac{2}{(2 x+1) x} y=0$ Fmd $P(x)=\frac{2 x+2}{(x+1) x}=\frac{A}{2 x+1}+\frac{B}{x}=\frac{A x+2 B x+B}{(2 x+1) x}$
$W=C e^{\left.-\int p( x\right) d x}=Ce^{-\int\left(\frac{-2}{2 x+1}+\frac{2}{x}\right) d x}=C e^{\int\left(\frac{2}{2 x+1}-\frac{2}{x}\right) d x}={Ce}^{\int \frac{2}{2 x+1} d x-\int \frac{2}{x} d x}=\operatorname{Ce}^{\ln| 2 x+1|-2 \ln | x |}=c \frac{e^{\ln |2 x+1|}}{e^{\sin |x|}}$
$W=\frac{2 x+1}{x^{2}}=\left|\begin{array}{ll}{y_{1}(x)} & {y_{2}(x)} \\ {y_{1}^{\prime}(x)} & {y_{2}^{\prime}(x)}\end{array}\right|=\left|\begin{array}{cc}{x+1} & {y_{2}(x)} \\ {1} & {y^{\prime}(x)}\end{array}\right|=(x+1) y_{2}^{\prime}(x)-y_{2}(x)$
$y_{1}(x)=x+1$
$y_{1}^{\prime}(x)=1$
$y_{1}^{\prime \prime}(x)=0$
$(2 x+1) x \cdot 0+(2 x+2) \cdot 1-2(x+1)=2 x+2-2 x-2=0$
$y_{2}^{\prime}(x)-\frac{1}{x+1} y_{2}(x)=\frac{2 x+1}{x^{2}(x+1)}$
$P(x)=-\frac{1}{x-4}$
$\mu=e^{\int p(x) d x}=e^{-\int \frac{1}{x+1} d x}=e^{-\ln |x+1|}=\frac{1}{x+1}$
$\frac{1}{x+1} y_{2}^{\prime}(x)-\frac{1}{(x+1)^{2}} y_{2}(x)=\frac{2 x+1}{x^{2}(x+1)^{2}}$
$\left[\frac{1}{x+1} y_{2}(x)\right]^{\prime}=\int \frac{2 x+1}{\left(x^{2}+x\right)^{2}} d x$
$\frac{1}{x+1} y_{2}(x)=\frac{-1}{x^{2}+x}$
$y_{2}(x)=\frac{-x-1}{x^{2}+x}=\frac{-1}{x}$
$y=C_{1} y_{1}+C_{2} y_{2}=C_{1}(x+1)+C_{2} \frac{-1}{x}=C_{1}(x+1)+C_{2} \frac{-1}{x}$
$y(-1)=1, \quad y=1, \quad x=-1$
$C_{1}(-1+1)+C_{2} \frac{-1}{-1}=1$
$C_{2}=1$
$y^{\prime}=C_{1}+\frac{C_2}{x^{-2}}$
$y^{\prime}(-1)=0$
$x=-1$
$y^{\prime}=0$
$0=C_{1}+\frac{C_2}{(-1)^{2}}=0$
$C_1+C_2=0$
$C_{1}=-1$
$y=-(x+1)+\frac{-1}{x}$
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