a是一元二次方程x2+3x-2的实数根
a^2+3a-2=0
a^2+3a=2
a-3/3a2-6a/[a+2-5/a-2]
=a-3/3a2-6a/[(a^2-4-5)/(a-2)]
=a-3/3a(a-2) *(a-2)/(a+3)(a-3)
=1/3a(a+3)
=1/3(a^2+3a)
=1/3*2
=1/6