先化简,后求值(1)5a2-3a+6-4a2+7a,其中a=-12.(2)2x-y-(2y2-x2)-5x+y+(x2+2y2),x=-1,y=1.-数学

题目简介

先化简,后求值(1)5a2-3a+6-4a2+7a,其中a=-12.(2)2x-y-(2y2-x2)-5x+y+(x2+2y2),x=-1,y=1.-数学

题目详情

先化简,后求值
(1)5a2-3a+6-4a2+7a,其中a=-
1
2

(2)2x-y-(2y2-x2)-5x+y+(x2+2y2),x=-1,y=1.
题型:解答题难度:中档来源:不详

答案

(1)5a2-3a+6-4a2+7a
=5a2-4a2+(-3a+7a)+6
=a2+4a+6,
当a=-class="stub"1
2
时,原式=(-class="stub"1
2
)2+4×(-class="stub"1
2
)+6=class="stub"1
4
-2+6=class="stub"17
4


(2)2x-y-(2y2-x2)-5x+y+(x2+2y2)
=2x-y-2y2+x2-5x+y+x2+2y2
=(2x-5x)+(-y+y)+(-2y2+2y2)+(x2+x2)
=-3x+2x2,
当x=-1,y=1时,原式=-3×(-1)+2×(-1)2=3+2=5.

更多内容推荐