lean_workbook_plus_76963

Use the Ravi Substitution: Since a, b, c are the lengths of a triangle, there are positive reals $x, y, z$ such that $a = y+z, b = z+x, c = x+y$ . Prove the inequality becomes: $\frac{x^3+y^3+z^3+x^2y+y^2z+z^2x-2(xy^2+yz^2+zx^2)}{(x+y)(y+z)(z+x)} \geq 0$