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Question

Let a and b be two unit vectors such that |a+b|=3. If c=a+2b+3(a×b), then 2|c| is equal to:

A
55
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B
51
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C
43
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D
37
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Solution

The correct option is A 55
|a+b|=3
|a+b|2=3
|a|2+2|a||b|cosθ+|b|2=3
cosθ=12θ=π3

c=a+2b+3(a×b)
|c|2=(a+2b+3(a×b))(a+2b+3(a×b))
|c|2=|a|2+4|b|2+9|a×b)|2+4ab
|c|2=1+4+9sin2π3+4×12
|c|2=554
|c|=552
2|c|=55

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