exam Maths Puzzle Numerical Ability

Please help me understand .
The equation is : ( a^-1 + b^-1)^2 . I need to simplify it. The answer is (b^2+2ab+a^2)/a^2*b^2.
Can you explain why. Why after using a common denominator , the " 2ab" is out of the fraction?
Thank you . Viktor

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