cho a,be0\overrightarrow{a},\overrightarrow{b}e0

CMR : aba+ba+b\left|\overrightarrow{a}\right|-\left|\overrightarrow{b}\right|\le\left|\overrightarrow{a}+\overrightarrow{b}\right|\le\left|\overrightarrow{a}\right|+\left|\overrightarrow{b}\right|

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