先化简,再求值:3(a-1)2-(2a+1)(a-2),其中a=−3/4.
问题描述:
先化简,再求值:3(a-1)2-(2a+1)(a-2),其中a=−
. 3 4
答
3(a-1)2-(2a+1)(a-2)
=3(a2-2a+1)-(2a2-4a+a-2)
=3a2-6a+3-2a2+3a+2
=a2-3a+5,
当a=-
时,原式=(-3 4
)2-3×(-3 4
)+5=3 4
+9 16
+5=9 4
.125 16