Let a=2 i- j+2 k and b= i+2 j- k. Let a vector v be in the plane containing a and b. If v is perpendicular to the vector 3 i+2 j- k and its…
JEE Main 2021 — Mathematics Vectors & 3D Geometry
2021integereasy
Let a=2i^−j^+2k^ and b=i^+2j^−k^. Let a vector v be in the plane containing a and b. If v is perpendicular to the vector 3i^+2j^−k^ and its projection on a is 19 units, then ∣2v∣2 is equal to _____.
Official previous-year question
Held on 1 Sept 2021 · Verified 6 Jul 2026.
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Solution
v,a,b are coplanar
v=λa+μb
v=(2λ+μ)i^+(2μ−λ)j^+(2λ−μ)k^
given v⊥(3i^+2j^−k^)
v⋅(3i^+2j^−k^)=0
⇒((2λ+μ)i^+(2μ−λ)j^+(2λ−μ)k^).(3i^+2j^−k^)=0
⇒3(2λ+μ)+2(2μ−λ)−(2λ−μ)=0
⇒λ=−4μ...(i)
and also v⋅a^=19⇒9λ−2μ=57...(ii)
solving (i) and (ii) we get
λ=6,μ=−23
⇒2v=21i^−18j^+27k^
⇒∣2v∣2=441+324+729
=1494
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