Quantrex Quantrex AcademyJEE · NEET · NDA PYQs with solutions Open app
JEE MainMathematicsVector Algebra

Let a vector a be coplanar with the vectors b = 3 i + 4 j and c = 8 i + 6 k . If the projection of a on b is equal to the projection of a on c , and the projection of a on the vector d = i - j + k is 2 3 , then a is equal to:

Options

  1. A14 i + 8 j + 6 k
  2. B6 13 (11 i + 4 j + 6 k )
  3. C7 i + 4 j + 3 k
  4. D1 3 (7 i + 4 j + 3 k )

Correct answer

C. 7 i + 4 j + 3 k

Step-by-step solution

Since a is coplanar with b and c , we can write a = x b + y c . The condition that the projection of a on b equals the projection of a on c gives: a b | b | = a c | c | We calculate the necessary dot products and magnitudes: | b | = 3^2 + 4^2 = 5 | c | = 8^2 + 6^2 = 10 b b = 25 c c = 100 b c = (3)(8) + (4)(0) + (0)(6) = 24 Substituting a = x b + y c into the projection equality: x( b b ) + y( b c ) 5 = x( b c ) + y( c c ) 10 25x + 24y 5 = 24x + 100y 10 2(25x + 24y) = 24x + 100y 50x + 48y = 24x + 100y 26x = 52y x =

Practice Vector Algebra on Quantrex Academy →

More from Vector Algebra

Let a , b be two vectors, and let P, Q and R be the points with position vectors a , b and a + b , respectively, with respect to the origin O . If | a + b | = 21 , | a - b | = 3 , 2026Let a = 4 i - j + 3 k , b = 10 i + 2 j - k and a vector c be such that 2( a b ) + 3( b c ) = 0 . If a c = 15 , then c ( i + j -3 k ) is equal to: 2026Let a = 2 i + 3 j + 3 k and b = 6 i + 3 j + 3 k . Then the square of the area of the triangle with adjacent sides determined by the vectors (2 a + 3 b ) and ( a - b ) is : 2026If a = i + j + k , b = j - k and c be three vectors such that a c = b and a c = 3 , then c ( a - 2 b ) is equal to _______. 2026Let O be the origin, OP = a and OQ = b . If R is the point on OP such that OP = 5 OR , and M is the point such that OQ = 5 RM , then PM is equal to : 2026Let a = 7 i + j - k and b = j + 2 k . If r is a vector such that r a + a b = 0 and r a = 0 , then |3 r |^2 is equal to: 2026Let u and v be unit vectors inclined at an acute angle such that | u v |= 3 2 . If A = u + v +( u v ) , then is equal to: 2026Let a_k = ( _k) i + j and b_k = i - ( _k) j , where _k = 2^ k-1 2^n + 1 , for some n N , n > 5 . Then the value of _ k=1 ^ n | a_k |^2 _ k=1 ^ n | b_k |^2 is _____. 2026 Full Vector Algebra list All JEE Main PYQs