(1)(a^2/a-1)-a-1
=[a^2-(a-1)(a+1)]/a-1
=(a^2-a^2+1)/a-1
=1/a-1
(2)[(a/a-2)-(a/a+2)]*(4-a^2/a)
=a(a+2-a+2)/(a^2-4)*(4-a^2/a)
=4a/(a^2-4)*(4-a^2/a)
=-4
(3)(1/2x)-(1/x+y)*[(x+y/2x)-x-y]
=(1/2x)-(1/x+y)*(x+y/2x)+(1/x+y)*(x+y)
=(1/2x)-(1/2x)+1
=1