Given $a, b​​, c, d>0$. ¿What is the necessary and sufficient condition so that it can form a convex quadrilateral with sides $a, b​​, c, d$?

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The necessary and sufficient condition is analogous to the one for the sides of a triangle, the triangle inequality: $a$, $b$, $c$ and $d$ can form a quadrilateral if and only if the sum of any three of them is greater than the fourth.