For a triangle ABC, let overrightarrow =overrightarrow BC, overrightarrow q=overrightarrow CA and overrightarrow =overrightarrow BA. If…
JEE Main 2026 — Mathematics Vectors & 3D Geometry
2026mcqhard
For a triangle ABC, let p=BC,q=CA and r=BA. If ∣p∣=23,∣q∣=2 and cosθ=31, where θ is the angle between p and q, then ∣p×(q−3r)∣2+3∣r∣2 is equal to :
Official previous-year question
Held on 21 Jan 2026 · Verified 6 Jul 2026.
Options
A
410
B
340
C
200
D
220
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Solution
Given ∣p∣=23, ∣q∣=2, and cosθ=31 where θ is the angle between p and q.
Since p=BC, q=CA, and r=BA, from triangle closure r=p+q.
p⋅q=∣p∣∣q∣cosθ=23⋅2⋅31=4
∣r∣2=∣p∣2+∣q∣2+2p⋅q=12+4+8=24
q−3r=q−3(p+q)=−3p−2q
p×(q−3r)=−2(p×q)
sin2θ=1−31=32
∣p×q∣2=12⋅4⋅32=32
∣p×(q−3r)∣2=4⋅32=128
Result: 128+3(24)=128+72=200
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