Question 68
$$
\sqrt{(x^2 + x y + y^2)(z^2 + z t + t^2)} + \sqrt{(y^2 - y z + z^2)(x^2 - x t + t^2)} \geq (x + z)(y + t).
$$
Find the cases of equality.
\sqrt{(x^2 + x y + y^2)(z^2 + z t + t^2)} + \sqrt{(y^2 - y z + z^2)(x^2 - x t + t^2)} \geq (x + z)(y + t).
$$
Find the cases of equality.