a是方程x^2+3x-6的根
则有a^2+3a-6=0
a^2+3a=6
（a^2-4a+4分之a^2-4—2-a分之1）÷a^2-2a分之2
=（ (a+2)(a-2)/(a-2)^2 +1/(a-2)）*(a^2-2a)/2
=（ (a+2)/(a-2) +1/(a-2)）*a(a-2)/2
= (a+3)/(a-2) *a(a-2)/2
= a(a+3)/2
= (a^2+3a)/2
=6/2
=3