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Question

Prove that (a+b)(a+b)=|a|2+b2 if and only if ab are perpendicular, given a0,b0

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Solution

To prove : (a+b)(a+b)=|a|2+|b|2
Given, a and b are perpendicular
Proof since a and b are perpendicular
therefore the dot product formula
a.b=|a||b|cos90o
[cos90o=0]
a.b=0
Now, LHS=(a+b).(a+b)
=a.a+a.b+b.a+b.b
=a.a+b.b[a.b=b.a=0]
|a|2+|b|2[a.a=|a|2]
=RHS
Hence proved

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