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.