先化简,再求值：当=1/2时,求15㎡-9-｛-4m²+[5m-8m²-(2m²-m)]-3m}的值

题目

先化简,再求值：当=1/2时,求15㎡-9-｛-4m²+[5m-8m²-(2m²-m)]-3m}的值

答案

原式=15m²-9+4m²-5m+8m²+2m²-m+3m
=15m²+4m²+8m²+2m²-5m-m+3m-9
=29m²-3m-9
=29/4-3/2-9
=29/4-21/2
=-13/4