把y=x+4代入y=1/2x^2得
1/2x^2=x+4
x^2-2x-8=0
(x-4)(x+2)=0
x=4 x=-2
所以面积S=
∫(-2->4)(x+4 -1/2x^2)dx
=∫(-2->4)xdx+4∫(-2->4)dx-1/2∫(-2->4)x^2dx
=1/2x^2|(-2->4)+4x|(-2->4)-1/6 x^3|(-2->4)
=1/2(4^2-2^2)+4(4-(-2))-1/6 * (4^3-(-2)^3)
=6+4*6-1/6*(64+8)
=6+24-12
=18