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Question

The lengths of the diagonals of a parallelogram constructed on the vectors p=2a+b & q=a2b, where a & b are unit vectors forming an angle of 60 are

A
3 & 4
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B
7 & 13
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C
5 & 11
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D
None
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Solution

The correct option is C 7 & 13
In the above parallelogram, the diagonals are given by p+q and pq.
Thus, the diagonals are 3ab and a+3b
Modulus of these vectors are respectively given by 9a26a.b+b2 & a2+6a.b+9b2
Since a & b are unit vectors, the modulii become 96×cos60o+1 & 1+6×cos60o+9
i.e. 7 and 13

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