Quantrex Quantrex AcademyJEE · NEET · NDA PYQs with solutions Open app
JEE Advanced 2018Paper 1
Exam
JEE Advanced
Year
2018
Paper
Paper 1
Subject
Mathematics
Chapter
Vector Algebra

JEE Advanced 2018 · Paper 1 · Mathematics · Vector Algebra

Let → a and → b be two unit vectors such that → a . → b = 0 . For...

Let a→and\overset{\to}{a} and\overset{\to}{b}betwounitvectorssuchthatbe two unit vectors such that\overset{\to}{a} . \overset{\to}{b} = 0 .ForsomeFor somex, y \in R,letlet\overset{\to}{c} = x\overset{\to}{a} + y\overset{\to}{b} + \left(\overset{\to}{a} \times \overset{\to}{b}\right) .If∣c→∣=2If \left|\overset{\to}{c}\right| = 2 and the vector c→\overset{\to}{c} is inclined at the same angle α\alpha to both a→andb→,\overset{\to}{a} and \overset{\to}{b} , then the value of 8cos⁡2α8\cos^{2} \alpha is

Correct answer

0

Step-by-step solution

\overset{\to}{c} = x\overset{\to}{a} + y\overset{\to}{b} + \overset{\to}{a} \times \overset{\to}{b}<br/><br />\overset{\to}{c} . \overset{\to}{a} = xandx = 2cos \alpha<br/><br />\overset{\to}{c} . \overset{\to}{b} = yandy = 2cos \alpha<br/>Also,∣a→×b→∣=1<br />Also, \left|\overset{\to}{a} \times \overset{\to}{b}\right| = 1
∴c→=2cosα(a→+b→)+a→×b→∴ \overset{\to}{c} = 2cos \alpha {\left(\overset{\to}{a} + \overset{\to}{b}\right) + \overset{\to}{a}}^{} \times \overset{\to}{b}
c→2=4cos⁡2α(a→+b→)2+(a→×b→)2+4cosα(a→+b→).(a→×b→)\overset{\to}{c}2 = 4\cos^{2} \alpha{\left(\overset{\to}{a} + \overset{\to}{b}\right)}^{2} + {\left(\overset{\to}{a} \times \overset{\to}{b}\right)}^{2} + 4cos \alpha\left(\overset{\to}{a} + \overset{\to}{b}\right) . \left(\overset{\to}{a} \times \overset{\to}{b}\right)
4=8cos⁡2α+14 = 8\cos^{2} \alpha + 1
8cos⁡2α=38\cos^{2} \alpha = 3
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… JEE Advanced 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… JEE Main 2026Let a = 2 i + 3 j + 3 k and b = 6 i + 3 j + 3 k . Then the square of the area of the… JEE Main 2026If a = i + j + k , b = j - k and c be three vectors such that a × c = b and a · c = 3… JEE Main 2026Let O be the origin, OP = a and OQ = b . If R is the point on OP such that OP = 5 OR… JEE Main 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… JEE Main 2026Let u and v be unit vectors inclined at an acute angle such that | u × v |= 3 2 . If A =λ… JEE Main 2026Let a_k = ( θ_k) i + j and b_k = i - ( θ_k) j , where θ_k = 2^ k-1 π 2^n + 1 , for some n… JEE Main 2026Let the vectors a = - i + j + 3 k and b = i + 3 j + k . For some λ, μ ∈ ℝ , let c = λ a +… JEE Main 2026Two adjacent sides of a parallelogram PQRS are given by PQ = j + k and PS = i - j . If… JEE Main 2026If a and b are two vectors such that | a | = 2 and | b | = 3 , then the maximum value of… JEE Main 2026Let P be a point in the plane of the vectors A B =3 i + j - k and A C = i - j +3 k such… JEE Main 2026For three unit vectors a , b , c satisfying | a - b |^ 2 +| b - c |^ 2 +| c - a |^ 2 =9… JEE Main 2026Let P Q R be a triangle such that P Q =-2 i - j +2 k and PR =a i +b j -4 k , a, b ∈ ℤ .… JEE Main 2026Let a =2 i -5 j +5 k and b = i - j +3 k . If c is a vector such that 2( a × c )+3( b × c… JEE Main 2026Let a =2 i - j - k , b = i +3 j - k and c =2 i + j +3 k . Let v be the vector in the… JEE Main 2026 Full Vector Algebra list Vector Algebra JEE Maths PYQsJEE Mathematics PYQs All JEE Advanced PYQs