Vector Addition in Linear Algebra: ||x+y||2 = ||x||2 + 2*x*y + ||y||2

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||x+y||2 = ||x||2 + 2*x*y + ||y||2
 
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||x+y||^2=(x+y).(x+y) (dot product). Expand it out.
 
Is there a question here? Are you trying to prove that relation for two vectors x and y?

If so, what have you tried?
 

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