a−b1+ab\frac{a-b}{1+a b}1+aba−b
a+b1−ab\frac{a+b}{1-a b}1−aba+b
2aba2+b2\frac{2 a b}{a^{2}+b^{2}}a2+b22ab
None of these \text { None of these } None of these