当0<=y<=1时，f(y)=2；当y<0或y>1时，f(y)=0当0<=x-y<=1，即x-1<=y<=x时，f(x-y)=2；当yx时，f(x-y)=0所以，当0<=y<=1且x-1<=y<=x时，f(y)f(x-y)=4；在其他区域，f(y)f(x-y)=0令D'={(x,y)|0<=y<=1且x-1<=y<=x}，平行四边形D'的面积=1原式=∫∫(D')4dxdy=4*1=4
