对任意向量$\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$，下列关系式中不恒成立的是（  ）

$\left|\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}\right|\le \left|\stackrel{\rightharpoonup }{a}\right|\left|\stackrel{\rightharpoonup }{b}\right|$ $\left|\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|\le |\left|\stackrel{\rightharpoonup }{a}\right|-\left|\stackrel{\rightharpoonup }{b}\right||$ $(\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}{)}^2={\left|\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}\right|}^2$ $(\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b})(\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b})={\stackrel{\rightharpoonup }{a}}^2-{\stackrel{\rightharpoonup }{b}}^2$

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