先化简,再求值!(a+2)(a-3)+2(a-6)(a+5)-3(a²-5a-17)其中a=-1/2

问题描述:

先化简,再求值!
(a+2)(a-3)+2(a-6)(a+5)-3(a²-5a-17)
其中a=-1/2

=a^2-a-6+2(a^2-a-30)-3a^2+15a+51
=a^2-a-6+2a^2-2a-60-3a^2+15a+51
=12a-15
=-6-15
=-21

原式=a²-a-6+2a²-2a-60-3a²+15a+51
=12a-15
代入a=-1/2,则=-21