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Question

Let e1 and e2 be unit vectors containing angle θ. If 12|e1e2|=sinλθ, then find λ.

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Solution

Given e1 and e2 are unit vector
|e1|=1=|e2|
By parallelogram law of vector addition
|a+b|=|a|2+|b|2+2|db|cosθ
|e1e2|=|e1|2+|e2|2+2|e1||e2|cos(180θ)
|e1e2|=1+12cosθ
|e1e2|=21cosθ
|e1e2|=22sin2θ/2
|e1e2|=22sinθ/2
|e1e2|=2sinθ/2
12|e1e2|=sinθ/2 .............. (1)
Comparing (1) with 12|e1e2|=sinθ
we get λ=12

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