JEE Main 2018MathematicsVector AlgebraMediumMCQ

JEE Main 2018 — Vector Algebra Question with Solution

JEE Main 2018 (15 Apr)

Question

If a→, b→, c→ are unit vectors such that a→+2b→+2c→=0→, then a→×c→ is equal to :

Choose an option

Show full solutionCorrect option: C
Correct answer
C154 

Step-by-step explanation

Given a→+2b→+2c→=0

⇒a→+2c→=-2b→

Squaring both sides,

⇒a→2+4c→2+4a→.c→=4b→2

⇒1+4+4cos⁡θ=4⇒cos⁡θ=-14

∴sin⁡θ=1-cos2⁡θ=154

Now, a→×c→=a→c→sin⁡θ=154

Practice this on the real CBT interface

Solve this JEE Main question (and the rest of the Vector Algebra chapter) on PrepSharp's TCS iON-style CBT player — with timer, bookmarks and session analytics.

Solve interactively →

About this question

This is a previous-year question from JEE Main 2018, covering the Vector Algebra chapter of Mathematics. PrepSharp catalogues every PYQ from JEE Main with a verified answer key and step-by-step solution prepared by IIT alumni — so you can search by chapter, topic or year and revise efficiently.