计算：（（3a-7)^2-(a+5)^2)/(4a-24),其中a=3.
问题描述：
计算：（（3a-7)^2-(a+5)^2)/(4a-24),其中a=3.
答
（（3a-7)^2-(a+5)^2)/(4a-24)，其中a=3.
=(3a-7+a+5)(3a-7-a-5)/4(a-6)
=4(2a-1)(a-6))/4(a-6)
=2a-1
=6-1
=5
答
（（3a-7)^2-(a+5)^2)/(4a-24)
=（3a-7+a+5)(3a-7-a-5)÷4(a-6)
=(4a-2)(2a-12)÷2(2a-12)
=2a-1
答
原式={[(3a-7)+(a+5)][(3a-7)-(a+5)]}/[4(a-6)]
=[4(a-6)(2a-1)]/[4(a-6)]
=2a-1
把a=3代入,2a-1=2*3-1=5
答
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
答
[（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
=2*3-1
=5