Quantrex Quantrex AcademyJEE · NEET · NDA PYQs with solutions Open app
JEE Main20204 Sep 2020Morning ShiftMathematicsVector AlgebraActual

Let x 0 be the point of local maxima of f x = a → · b → × c → , where a → = x i ^ - 2 j ^ + 3 k ^ , b → = - 2 i ^ + x j ^ - k ^ and c → = 7 i ^ - 2 j ^ + x k ^ . Then the value of a → · b → + b → · c → + c → · a → at x = x 0 is:

Options

  1. A- 4
  2. B- 30
  3. C14
  4. D- 22

Correct answer

D. - 22

Step-by-step solution

f x = a →    b →    c → = x         − 2          3 − 2          x        − 1 7           − 2          x = x x 2 − 2 + 2 − 2 x + 7 + 3 4 − 7 x = x 3 − 2 x − 4 x + 14 + 12 − 21 x f x = x 3 − 27 x + 26 f '

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