Áp dụng BĐT Cauchy-Schwarz ta có:
\(\left(1^2+1^2\right)\left(x^2+y^2\right)\ge\left(x+y\right)^2\)
\(\Rightarrow2\left(x^2+y^2\right)\ge\left(x+y\right)^2\)
\(\Rightarrow2\cdot1=2\ge\left(x+y\right)^2\)
Đẳng thức xảy ra khi \(x=y=\pm\dfrac{1}{\sqrt{2}}\)