InterviewSolution
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If `veca+vecb+vecc=vec0, |veca| = 3, |vecb| = 5, |vecc| = 7`, then angle between `veca` and `vecb` isA. ` ( pi ) /( 2 ) `B. ` ( pi ) /( 3 ) `C. ` ( pi ) /( 4 ) `D. ` ( pi ) /( 6 ) ` |
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Answer» Correct Answer - B Given , a + b + c = 0 and ` |a| = 5, |b| = 3, |c| = 7 ` ` rArr a + b = - c ` On squaring both sides, we get ` (a + b ) ^ 2 = ( - c ) ^ 2 ` `rArr |a + b| ^ 2 = |c| ^ 2 ` ` rArr |a| ^ 2 + |b| ^ 2 + 2a * b = |c| ^ 2 ` ` ( because theta ` be the angle between a and b ) ` rArr ( 5 ) ^ 2 + ( 3 ) ^ 2 + 2| a||b| cos theta = ( 7 ) ^ 2 ` ` rArr 25 + 9 + 2* 5 * 3 * cos theta = 49` ` rArr 30 cos theta = 15 ` ` rArr cos theta = ( 1 ) / ( 2 ) = cos 60 ^@ ` `rArr theta = ( pi) / ( 3 ) ` |
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