解:(x+2y)dx+(2x-y)dy=d(0.5x²+2xy-0.5y²)
∂P/∂y = 2 = ∂Q/∂xu = ∫<0, x>(x+2y)dx + ∫<0, y>(0-y)dy = x^2/2+ 2xy - y^2/2 = C
