InterviewSolution
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Find a vector of magnitude 9, which is perpendicular to both vectors `4 hat i- hat j+3 hat ka n d-2 hat i+ hat j-2 hat k`.A. `3i + 6j - 6 k `B. ` 3i - 6j + 6k `C. `-3i + 6j + 6k `D. None of the above |
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Answer» Correct Answer - C Let ` a = 4i - j + 3k, b = - 2i + j - 2 k ` and ` c =x i + yj + zk ` Given ` a * c = 0 ` i.e., ` 4 x -y + 3z= 0 " " `… (i) and ` b * c = 0 ` i.e., ` - 2x + y -2z = 0 " " `...(ii ) Also, `|c | = 9 ` i.e., ` x ^ 2 + y ^ 2 + z ^ 2 = 81 " " `... (iii) Now, from Eqs. (i) and (ii), we get ` 2x + z = 0 rArr z = - 2x ` On putting this value in Eq. (iii), we get ` x ^ 2 + y ^ 2 + 4 x^ 2 = 81 ` ` rArr 5x^ 2 + y ^ 2 = 81 " " `... (iv) On multiplying Eq. (i) by 2 and Eq. (ii) by 3 and then adding , we get ` {:( 8x - 2y + 6 z= 0 ), (ul(-6x + 3y - 6z = 0 )), ( 2x + y = 0 ) :} ` ` rArr y = - 2x ` On putting this value in Eq. (iv) , we get ` 5x ^ 2 + 4x ^ 2 = 8 1 ` ` rArr 9 x ^ 2 = 81 ` ` rArr x ^ 2 = 9 ` ` rArr x = pm 3 ` ` therefore y = pm 6 and z = pm 6 ` ` therefore ` Required vector, ` c = x i + yj + zk ` ` = pm 3i pm 6j pm 6 k ` ` = 3i - 6 j - 6k ` ` = -3i + 6j + 6k` |
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