If {a^2} + {b^2} = 4 and {\left( {a - b} \right)^2} = 2 , what is the value of {ab} ?

Question

I don't know how to start this problem

Answer 1

 

Hope you know (a+b)^2 = a^2+b^2+2ab  

  

        and 

 

  (a-b)^2 = a^2 +b^2 -2ab 

   

 2 =  4 -2ab 

 2ab = 4-2 --> 2ab = 2 --> ab =1