Question

先化简,再求值．
(1)4(2m﹣n)﹣7(m﹣n),其中．
(2),其中x=﹣2,y=﹣1．

Answer 1

解:(1)原式=8m﹣4n﹣7m+7n
=m+3n,
当时,原式=；
(2)原式=x²y﹣2(x²y﹣xy²)+(x²y﹣xy²)﹣3
=x²y﹣2x²y+xy2﹣x²y﹣xy²﹣3
=﹣3x²y﹣3
当x=﹣2,y=﹣1时,原式=9