Sometimes an equation that is not linear in y is linear in x. Example: dy/dx = $\frac{1}{x + y^2}$. This is NOT linear in y (nonlinear term $y^2$ multiplies nothing). But rewrite as dx/dy = x + $y^2$, i.e., dx/dy - x = $y^2$. This IS linear in x with P(y) = -1 and Q(y) = $y^2$. IF = e^(-y). Solution: x*e^(-y) = integral of $y^2$*e^(-y) dy + C. This trick is crucial in JEE — always consider whether switching dependent and independent variables simplifies the equation. The hint is usually that dy/dx has a complex expression but dx/dy is simpler.