1，u=(x+y)^(1/2) ∂u/∂x=(1/2)(x+y)^(1/2-1)=(1/2)/(x+y)^(1/2) ∂u/∂y=(1/2)(x+y)^(1/2-1)=(1/2)/(x+y)^(1/2)2，∫ x^(1/2) dx=1/(1+1/2)x^(1+1/2)+c=(2/3)x^(3/2)+c
