a^2-1分之2-a-1分之1
=1/(a+1)(a-1)-1/(a-1)
=(1-a-1)/(a+1)(a-1)
=-a/(a+1)(a-1) 带入a=0
=0/(0+1)(0-1)
=0a^2-1分之2=1/(a+1)(a-1)？