<s>[INST] Proving $\sqrt{a^2+c^2}+\sqrt{b^2+d^2}\ge \:\sqrt{\left(a+b\right)^2+\left(c+d\right)^2}$ The inequality:
$$\sqrt{a^2+c^2}+\sqrt{b^2+d^2}\ge \:\sqrt{\left(a+b\right)^2+\left(c+d\right)^2}$$
But can someone help me with a nice elegant solution. This is an olympiad question I was trying to solve, but couldn't manage an elegant solution.
 [/INST] Since squaring is monotonic, this is equivalent to
$$
a^2 + b^2 + c^2 + d^2 + 2\sqrt{a^2+c^2}\sqrt{b^2+d^2} \ge a^2+b^2+c^2+d^2 + 2(ab + cd)
$$
which is in turn equivalent to
$$
ab + cd \le \sqrt{a^2+c^2}\sqrt{b^2+d^2},
$$
the Cauchy-Schwarz inequality.
Putting it in vector form is even more elegant. For $\mathbf{a} = (a,c)$ and $\mathbf{b} = (b,d)$, this is
$$
|\mathbf{a}| + |\mathbf{b}| \ge |\mathbf{a}+\mathbf{b}|,
$$
the triangle inequality.
</s>