（1）a-2分之4+2-a分之a+2
=4/(a-2)+(-a-2)/(a-2)
=(2-a)/(a-2)
=-1
(2)a+3分之2+a平方-9分之7-2a
=2/(a+3)+(7-2a)/(a+3)(a-3)
=(2a-6)/(a+3)(a-3)+(7-2a)/(a+3)(a-3)
=1/(a^2-9)