dy/dx + y/x = cosx
积分因子 = e^∫ 1/x dx = e^ln|x| = x,乘以方程两边
x · dy/dx + y = xcosx
d(xy)/dx = xcosx
xy = ∫ xcosx dx
xy = ∫ x d(sinx) = xsinx - ∫ sinx dx = xsinx + cosx + C
y = sinx + (cosx)/x + C/x