(a+b)/ab*2ab/(a+b)(a-b)=2/(a-b)
2. (m+n-m+n)/2(m-n)(m+n)*3(m-n)=
2n*3(m-n)
= ---------------- = 3n/(m+n)
2(m-n)(m+n)
3. a(a-5)
--------------- = a/(a+1) a-a/(a+1) =(a²+a-a)/(a+1)=a²/(a+1)
(a+1)(a-5)
4. (x+y)(x-y) 3y 1
---------------*---------*---------=3/x
xy (x-y) (x+y)