The harmonic mean of two numbers aaa and bbb is HHH. What is the formula for HHH?
a+b2\frac{a+b}{2}2a+b
21a+1b\frac{2}{\frac{1}{a} + \frac{1}{b}}a1+b12
ab\sqrt{ab}ab
aba+b\frac{ab}{a+b}a+bab