(1/2)(x+5)(x+6)=28
(x+5)(x+6)=56
x²+11x+30=56
x²+11x-26=0 [ 用十字相乘,(-2)x13=26 (-2)+13=11]
(x+13)(x-2)=0
x1=-13
x2=2
1/2(x+5)(x+6)=28
解:(x+5)(x+6)=28×2
x^2+11x+30=56
x^2+11x=26
剩下的自己写吧,不能全抄