平面上 $O,A,B$三点不共线，设 $\stackrel{\rightharpoonup }{OA}=a,\stackrel{\rightharpoonup }{OB}=b$,则 $△OAB$的面积等于

A．	$\sqrt{{\left|a\right|}^2+{\left|b\right|}^2-(\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}{)}^2}$	B．	$\sqrt{{\left|a\right|}^2+{\left|b\right|}^2+(\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}{)}^2}$
C．	$\frac{1}{2}\sqrt{{\left|a\right|}^2+{\left|b\right|}^2-(\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}{)}^2}$	D．	$\frac{1}{2}\sqrt{{\left|a\right|}^2+{\left|b\right|}^2+(\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}{)}^2}$