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Question

If a,b and c are unit vectors satisfying |ab|2+|bc|2+|ca|2=9, then |2a+5b+5c| is

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Solution

Given that,
ab2+bc2+ca2=9
2(a2+b2+c2)2(ab+bc+ca)=92(1+1+1)2(ab+bc+ca)=92×32(ab+bc+ca)=962(ab+bc+ca)=9ab+bc+ca=32.........(1)

Let A=2a+5b+5c
=4+25+25+2(2a.5b+5b.5c+5c.2a)=54+2(10ab+25bc+10ca)=54+2(10(ab+bc+ca)+15bc)
=54+2(10(32)+15bc)=2430+30bc=24+30bc

Since, b.c1

Therefore,
A=24+30
A=54

Hence, this is the answer.

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