$dpcm\Leftrightarrow \sqrt[n]{\frac{x_{1}x_{2}...x_{n}}{(x_{1}+y_{1})(x_{2}+y_{2})...(x_{n}+y_{n})}}+\sqrt[n]{\frac{y_{1}y_{2}...y_{n}}{(x_{1}+y_{1})(x_{2}+y_{2})...(x_{n}+y_{n})}}\leq 1$
Áp dụng AM-GM ta có:
$\sqrt[n]{\frac{x_{1}x_{2}...x_{n}}{(x_{1}+y_{1})(x_{2}+y_{2})...(x_{n}+y_{n})}}\leq \frac{1}{n}\left ( \frac{x_{1}}{x_{1}+y_{1}} +\frac{x_{2}}{x_{2}+y_{2}}+...+\frac{x_{n}}{x_{n}+y_{n}}\right )$
$\sqrt[n]{\frac{y_{1}y_{2}...y_{n}}{(x_{1}+y_{1})(x_{2}+y_{2})...(x_{n}+y_{n})}}\leq \frac{1}{n}\left ( \frac{y_{1}}{x_{1}+y_{1}} +\frac{y_{2}}{x_{2}+y_{2}}+...+\frac{y_{n}}{x_{n}+y_{n}}\right )$
Cộng 2 BĐT trên ta có đpcm