1/a-1/b=b/ab-a/ab=(b-a)/ab
(x^2+4x+4)/(x^2-4)-x/(x-2)=(x+2)^2/(x+2)(x-2)-x/(x-2)=(x+2)/(x-2)-x/(x-2)=2/(x-2)
1/b+2/b=(1+2)/b=3/b