先化简再求值 [(1/x^2-7x+12)+(2/x^2-4x+3)]/(3x/x^2-5x+4),其中x=-1/7
问题描述:
先化简再求值 [(1/x^2-7x+12)+(2/x^2-4x+3)]/(3x/x^2-5x+4),其中x=-1/7
答
原式=[1/(x-3)(x-4) +2/(x-1)(x-3) ]×(x-4)(x-1)/3x
=[(x-1)+2(x-4)]/(x-1)(x-3)(x-4) ×(x-4)(x-1)/3x
=(3x-9)/(x-3) ×1/3x
=3(x-3)/(x-3) ×1/3x
=1/x