因式分解:(2x+5)(x^2-9)(2x-7)-91 要用换元法!

问题描述:

因式分解:(2x+5)(x^2-9)(2x-7)-91 要用换元法!

(2x-7)(2x+5)(x^2-9)-91
=(2x-7)(x+3)(2x+5)(x-3)-91
=(2x^2-x-21)( 2x^2-x-15)-91
设 2x^2-x=a
原式=(a-21)(a-15)-91
=a²-36a+315-91
=a²-36a+224
=(a-28)(a-8)
原式等于(2x^2-x-28)(2x^2-x-8)

(2x+5)(x^2-9)(2x-7)-91 =(2x+5)(x-3)(x+3)(2x-7)-91=[(2x+5)(x-3)][(x+3)(2x-7)]-91=(2x^2-x-15)(2x^2-x-21)-91设2x^2-x-15=y则原式=y(y-6)-91=y^2-6y-91=(y-13)(y+7)所以原式=(x^2-x-15-13)(x^...