Question 743374
Omg, he is clearly asking you to prove it, duh. You are clearly not an Algebra master if you don't even know this simple identity, I think you are some kind of troller.

By the Cauchy-Schwarz Inequality we have:

(a+b+c+d)^2<=(1+1+1+1)(a^2+b^2+c^2+d^2)