Let a= i+ j+2 k, b=2 i-3 j+ k and = i- j+ k be the three given vectors. Let v be a vector in the plane of a and b whose projection on is 2 √3. If v,…
JEE Main 2022 — Mathematics Vectors & 3D Geometry
2022mcqeasy
Let a=i^+j^+2k^,b=2i^−3j^+k^ and c=i^−j^+k^ be the three given vectors. Let v be a vector in the plane of a and b whose projection on c is 32. If v,j^=7, then v⋅(i^+k^) is equal to
Official previous-year question
Held on 26 Jun 2022 · Verified 6 Jul 2026.
Options
A
6
B
7
C
8
D
9
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Solution
Since vector \vec{v},\vec{a}&\vec{b} are coplanar then, v=λ1a+λ2b, where λ1,λ2∈R.
⇒v=(λ1+2λ2)i^+(λ1−3λ2)j^+(2λ1+λ2)k^
∵ Projection of v on c is 32
∴3λ1+2λ2−λ1+3λ2+2λ1+λ2=32
∴λ1+3λ2=1...(1)
and v⋅j^=7⇒λ1−3λ2=7...(2)
⇒v=(λ1+2λ2)i^+(λ1−3λ2)j^+(2λ1+λ2)k^
∴λ1+3λ2=1...(1)
⇒λ1−3λ2=7(2)
From equation (1) and (2), we get
λ1=4,λ2=−1
∴v=2i^+7j^+7k^
∴v⋅(i^+k^)=2+7=9
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