Question : If $a^3-b^3=56$ and $a-b=2$, what is the value of $(a^2+b^2)$?
Option 1: 12
Option 2: 20
Option 3: 28
Option 4: 32
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Correct Answer: 20
Solution :
Given:
$a^3-b^3=56$ and $a-b=2$
Now, $(a-b)^3=a^3-b^3-3ab(a-b)$
⇒ $2^3=56-3ab×2$
⇒ $6ab=48$
⇒ $ab=8$
Now, $(a-b)^2=a^2+b^2-2ab$
⇒ $2^2=a^2+b^2-2×8$
⇒ $a^2+b^2=4+16$
$\therefore a^2+b^2=20$
Hence, the correct answer is 20.
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