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Question

If a,b,c are vectors of lengths 2,3,4. If a is orthogonal to b+c,b is orthogonal to c+a and c is orthogonal to a+b, then the modulus of a+b+c is

A
29
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B
39
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C
45
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D
219
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Solution

The correct option is A 29
¯¯¯a.(¯¯b+¯¯c)=0 ----(1)
¯¯b.(¯¯c+¯¯¯a)=0 ----(2)
¯¯c.(¯¯¯a+¯¯b)=0 ----(3)

adding (1)+(2)+(3)
we get 2(¯¯¯a.¯¯b+¯¯b.¯¯c+¯¯c.¯¯¯a)=0
So, |¯¯¯a+¯¯b+¯¯c|=|¯¯¯a|2+|¯¯b|2+|¯¯c|22(¯¯¯a¯¯b+¯¯b¯¯c+¯¯c¯¯¯a)
=4+9+160
=29

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