ISL 1976 Problem 46

Let $a,b,c,d$ be nonnegative real numbers. Prove that $a^4+b^4+c^4+d^4+2abcd \ge a^2b^2+a^2c^2+a^2d^2+b^2c^2+b^2d^2+c^2d^2.$

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Solutions

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