Question 1165795
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Using properties and definition of the scalar product of vectors, you have


  ||a+b||^2 = ||a||^2 + 2*(a,b) + ||b||^2

  ||a-b||^2 = ||a||^2 - 2*(a,b) + ||b||^2


Now add these two identities, and you will get the statement which has to be proved.
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Solved.