($\frac{1}{a}$ + $\frac{1}{b}$).( a + b ) $\geq$ 4.$\sqrt[]{\frac{1}{a}\frac{1}{b}.a . b}$ =>($\frac{1}{a}$ + $\frac{1}{b}$).( a + b ) $\geq$ 4.$\sqrt[]{\frac{a}{a}\frac{b}{b}}$ =>($\frac{1}{a}$ + $\frac{1}{b}$).( a + b ) $\geq$ 4.1 => ($\frac{1}{a}$ + $\frac{1}{b}$).( a + b ) $\geq$ 4
