Sometimes an equation that is not linear in y is linear in x. Example: dy/dx = 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: xe^(-y) = integral of y^2e^(-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.