问题
计算题
先化简,再求值(x﹣5y)(﹣x﹣5y)﹣(﹣x+5y)2,其中x=﹣1,y=0.5.
答案
解:(x﹣5y)(﹣x﹣5y)﹣(﹣x+5y)2
=﹣(x2﹣25y2)﹣(x2﹣10xy+25y2)
=﹣x2+25y2﹣x2+10xy﹣25y2
=(﹣1﹣1)x2+(25﹣25)y2+10xy
=﹣2x2+10xy,
∵x=﹣1,y=0.5,
∴上式=﹣2×(﹣1)2+10×(﹣1)×0.5
=﹣2﹣5
=﹣7.