{a/(a-1)-2/(a^2*1)}÷{1-1/(a+1)}
={a(a+1)/(a^2-1)-2/(a^2*1)}÷{a/(a+1)}
={(a^2+a-2)/(a^2-1)}÷{a/(a+1)}
={(a+2)(a-1)/(a^2-1)}÷{a/(a+1)}
={(a+2)/(a+1)}÷{a/(a+1)}
=(a+2)/(a+1)*(a+1)/a
=(a+2)/a
=1+2/a
令a=2
原式=1+2/a=1+2/2=1+1=2