问题 计算题

先化简后求值: (1)3(3x2+4﹣5x3)﹣2(x3﹣3+3x2),其中x=﹣1;

(2)(3a3﹣a2+a﹣1)﹣[4a3+2a2﹣3(a+2)],其中a=﹣1.

答案

解:(1)3(3x2+4﹣5x3)﹣2(x3﹣3+3x2

=9x2+12﹣15x3﹣2x3+6﹣6x2

=﹣17x3+3x2+18

当x=﹣1时,

原式=﹣17x3+3x2+18

=﹣17×(﹣1)3+3×(﹣1)2+18

=17+3+18=38;

(2)(3a3﹣a2+a﹣1)﹣[4a3+2a2﹣3(a+2)]

=3a3﹣a2+a﹣1﹣4a3﹣2a2+3(a+2)

=﹣a3﹣3a2+a﹣1+3a+6

=﹣a3﹣3a2+4a+5

当a=﹣1时,

原式=﹣a3﹣3a2+4a+5

=﹣(﹣1)3﹣3×(﹣1)2+4×(﹣1)+5

=1﹣3﹣4+5=6﹣7=﹣1.

判断题
单项选择题