先化简再求值：（x-1)(x+2)+(2x-1)(x+5)-3(x的平方-6x-1),其中x=7/2
抱歉，还有几题
2(x-y)^2-(y-x)^2-(x+y)(y-x); x=3,y=-2;
(3/4a^4b^7+1/2a^3b^8-1/9a^2b^6)/(-1/3ab)^2; a=1/2,b=-4;
[(-2xy^2)^2x^3-2x^2(2xy^2)^3*1/2y]/4x^4y^2 x=1/2,y=-1;