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

A．	$\left|\stackrel{\rightharpoonup }{a}\stackrel{\rightharpoonup }{b}\right|\le \left|\stackrel{\rightharpoonup }{a}\right|\left|\stackrel{\rightharpoonup }{b}\right|$
B．	$\left|\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|\le \left|\left|\stackrel{\rightharpoonup }{a}\right|-\left|\stackrel{\rightharpoonup }{b}\right|\right|$
C．	${\left(\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}\right)}^2={\left|\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b^{}}\right|}^2$
D．	$\left(\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}\right)\left(\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right)=\stackrel{\rightharpoonup }{a^2}-\stackrel{\rightharpoonup }{b^2}$

科目 数学   题型 选择题   难度 较易