It is known that (a+b)**2=a**2+2ab+b**2, (ab)**2=a**2-2ab+b2, when (a+b)**2=28 , (ab)**2=12, then ab=how much.
已知(a+b)**2=a**2+2ab+b**2，(a-b)**2=a**2-2ab+b2，当(a+b)**2=28，(a-b)**2=12时，则ab=多少．
<gadget id="calculator">2 + 2</gadget>
<output>4</output>

<result>4</result>