Ta có:\(\left(\left|a\right|+\left|b\right|\right)^2-\left(\left|a+b\right|\right)^2\)\(=\left|a\right|^2+\left|b\right|^2+2\left|a\right|\left|b\right|-\left|a\right|^2-\left|b\right|^2-2ab\)
\(=2\left|a\right|\left|b\right|-2ab\ge0\)
=>\(\left|a\right|+\left|b\right|\ge\left|a+b\right|\)
Dấu "="xảy ra khi |a|=a,|b|=b khi \(a\ge0,b\ge0\)