Toronto Math Forum
MAT2442019F => MAT244Test & Quizzes => Quiz2 => Topic started by: Jiayue Wu on October 04, 2019, 06:25:19 PM

show that the given equation is not exact but becomes exact
when multiplied by the given integrating factor. Then solve the equation.
$$x^2y^3+x(1+y^2)y'=0, u(x.y)=\frac{1}{xy^3}$$
Solution:
This is not exact because
$$\frac{\partial}{\partial y}(x^2y^3)=3x^2y^2 \neq \frac{\partial}{\partial x}[x(1+y^2)]=q=y^2$$
However, multiplyin by $\mu$, we get: $$x+\frac{1+y^2}{y^3}y'=0$$
it is exact since $$\frac{\partial}{\partial y}(x) = \frac{\partial}{\partial x}(\frac{1+y^2}{y^3})=0$$
Next, I will find the function $\phi$.
$$\frac{\partial \phi}{\partial x}= x$$
$$\phi = \int x dx = \frac{x^2}{2}+h(y)$$
$$\frac{\partial \phi}{\partial y} = h'(y) = \frac{1+y^2}{y^3}$$
$$h(y) = \int \frac{1+y^2}{y^3} dy = \frac{1}{2y^2} + lny$$
As a result, the function is $$\phi (x,y) = \frac{x^2}{2}\frac{1}{2y^2}+lny$$