Áp dụng bđt: |A + B| ≤ |A| + |B|
Ta có: |A + B| + |B| ≥ |(A - B) + B| = |A|
=> |A + B| ≥ |A| - |B|
SOS thử xem:)
\(\left|a-b\right|\ge\left|a\right|-\left|b\right|\)
\(\Leftrightarrow\left(a-b\right)^2\ge\left(\left|a\right|-\left|b\right|\right)^2\)
\(\Leftrightarrow a^2-2ab+b^2\ge a^2-2\left|ab\right|+b^2\)
\(\Leftrightarrow2ab\le2\left|ab\right|\left(Q.E.D\right)\)