(3) \(\frac1x\frac{d^2y}{dx^2}=e^x\)
⇒ \(\frac{d^2y}{dx^2}=xe^x\)
By comparing with \(\frac{d^2y}{dx^2}+p(x)\frac{dy}{dx}+q(x,y)=f(x),\)
we get p(x) = 0, q(x) = 0 f(x) = xex
∴ This is linear equation of second order.
(4) (xy2 + x)dx + (y - x2y)dy = 0
⇒ \(\frac{dy}{dx} = \frac{xy^2+x}{x^2y-y}\) which is not a linear equation.