If a - b = 3 and a^2 + b^2 = 31, find the value of ab. Choose the correct choice from the following choices:  A) 12  B) 15  C) 10  D) 11  E) 13
2ab = (a^2 + b^2) - (a - b)^2   = 31 - 9= 
<gadget id="calculator">31 - 9</gadget>
<output>22</output>
22   ab= 11.   Answer is 11.
<result>D</result>
If a - b = 3 and a^2 + b^2 = 31, find the value of ab.