JEE Main 2021 · MathsMultiple choiceSingle correctMediumCalculation

JEE Main 31 August 2021, Shift 1, Maths Q78

Question 78 of 90 in this shift, Maths question 18 of 30, Section A.

Let

\vec{\mathrm{a}}

and

\vec{\mathrm{b}}

be two vectors such that

|2\vec{\mathrm{a}}+3\vec{\mathrm{b}}|=|3\vec{\mathrm{a}}+\vec{\mathrm{b}}|

and the angle between

\vec{\mathrm{a}}

and

\vec{\mathrm{b}}

60^\circ

. If

\frac{1}{8}\vec{\mathrm{a}}

is a unit vector, then

|\vec{\mathrm{b}}|

is equal to :

(1) $4$
(2) $5$ Official answer
(3) $6$
(4) $8$

Official answer

Option 2

NTA final key.

Chapter: Vector Algebra
Topic: Dot Product
Idea tested: Expansion of Sum of Vectors Squared

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Asked 4× in all

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Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.