Question : If $a-b=-7$ and $a^2+b^2=85$, then find $ab$.
Option 1: 18
Option 2: –30
Option 3: –44
Option 4: –60
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Correct Answer: 18
Solution :
We know the algebraic identity, $(a-b)^{2} = a^{2}+b^{2}-2ab$.
By substituting the values of $a-b = -7$ and $a^{2}+b^{2} = 85$ in the above equation, we get,
$⇒(–7)^{2} = 85-2ab$
$⇒49 = 85-2ab$
$⇒-36 = -2ab$
$\therefore ab=18$
Hence, the correct answer is 18
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