P(x)y" + Q(x)y' + R(x)y = 0 is said to be exact if it can be written in the form (P(x)y')' + (f(x)y)' = 0, where f(x) is to be determined in terms of P(x), Q(x), and R(x). The latter equation can be integrated once immediately, resulting in a first-order linear equation for y that can be solved easily. (a) By equating the coefficients of the preceding equations and then eliminating f(x), show that the equation is exact if P" (x) - Q'(x) + R(x) = 0. (b). By above observations solve the equation xy" — (cos x)y' + (sin x)y = 0, x > 0