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Question

If a+b+c=0,|a|=3,|b|=5,|c|=7, then the angle between a and b is

A
5π3
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B
π6
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C
π3
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D
2π3
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Solution

The correct option is C π3
a+b+c=0,|a|=3,|b|=5,|c|=7
(a+b+c)2=0
|a|2+|b|2+|c|2+2(a.b+b.c+c.a)=0
a.b+b.c+c.a=832 ... (1)
a.a+a.b+a.c=0a.b+a.c=9 ... (2)
a.b+b.b+b.c=0a.b+b.c=25 ... (3)
2(a.b)+a.c+b.c=34a.b=152
Angle between a and b is π3

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