y'+x^2y=x^2
dy/dx=x^2(1-y)
dy/(1-y)=x^2dx
-d(1-y)/(1-y)=x^2dx
-ln|1-y|=x^3/3+C1
(1-y)=Ce^(-x^3/3)
通解是y=1-Ce^(-x^3/3)
特解C=0
y=1