abcd为不全相等的正数,求证1/a^2+1/b^2+1/c^2+1/d^2>1/ab+1/bc+1/cd+1/da

问题描述:

abcd为不全相等的正数,求证1/a^2+1/b^2+1/c^2+1/d^2>1/ab+1/bc+1/cd+1/da