主要用到平方差公式：x²-y²=(x+y)(x-y) ((3a-7)^2-(a+5)^2)/(4a-24) =(3a-7+a+5)(3a-7-a-5)/(4(a-6)) =(4a-2)(2a-12)/(4(a-6)) =4(2a-1)(a-6)/(4(a-6)) =2a-1 当a=3 原式=2×3-1=5