（2a＋5）（a＾2－9）（2a－7）－91
參考答案:有一类题的变形处理都是这样的
(2a+5)(a^2-9)(2a-7)-91
= (2a+5)(a-3)(a+3)(2a-7)-91
= [(2a+5)(a-3)][(a+3)(2a-7)]-91
= (2a^2-a-15)(2a^2-a-21)-91
= (2a^2-a)^2-36(2a^2-a)+224
= (2a^2-a-8)(2a^2-a-28)
= (2a^2-a-8)(2a+7)(a-4)
如果是在实数范围内分解
= 2(a-0.25(1+sqrt(65)))(a-0.25(1-sqrt(65)))(2a+7)(a-4)
