Question Details

Let w = î + ĵ − 2k̂, and u and v be two vectors such that u × v = w and v × w = u. Let α, β, γ, and t be real numbers such that

u = αî + βĵ + γk̂, −tα + β + γ = 0, α − tβ + γ = 0, and α + β − tγ = 0.


Match each entry in List-I to the correct entry in List-II and choose the correct option.

List-I List-II
(P) |v|² is equal to (1) 0
(Q) If α = √3, then γ² is equal to (2) 1
(R) If α = √3, then (β + γ)² is equal to (3) 2
(S) If α = √2, then t + 3 is equal to (4) 3

(5) 5

Options

A

(P) → (2), (Q) → (1), (R) → (4), (S) → (5)

B

(P) → (2), (Q) → (4), (R) → (3), (S) → (5)

C

(P) → (2), (Q) → (1), (R) → (4), (S) → (3)

D

(P) → (5), (Q) → (4), (R) → (1), (S) → (3)

Show Answer

Correct Answer :

Option A

(P) → (2), (Q) → (1), (R) → (4), (S) → (5)

Solution :

The correct option is (P) → (2), (Q) → (1), (R) → (4), (S) → (5).

Given:
w=i+j-2k
And two vectors u and v satisfy:
1) u×v=w
2) v×w=u

From equation (1), w is perpendicular to both u and v. Therefore, the angle between u and v is 90°.
Taking the magnitude of equation (1):
|w|=|u||v|sin(90°)=|u||v|
Similarly, from equation (2), since v is perpendicular to w, taking the magnitude gives:
|u|=|v||w|
Substituting |u| into the first magnitude equation:
|w|=(|v||w)|v|=|v|2|w|
Since w0, we have:
|v|2=1
This matches (P) → (2).

Next, the magnitude of w is:
|w|=12+12+(-2)2=6
Thus, the magnitude of u is:
|u|=|v||w|=1·6=6
Since u=αi+βj+γk, we have:
α2+β2+γ2=6

We are given a system of homogeneous linear equations in α,β,γ:
-tα+β+γ=0
α-tβ+γ=0
α+β-tγ=0
For a non-trivial solution (since |u|=60), the determinant of the coefficient matrix must be zero:
-t111-t111-t=0
Expanding this determinant:
-t(t2-1)-1(-t-1)+1(1+t)=0
-t3+3t+2=0
t3-3t-2=0
Factoring this cubic equation gives:
(t+1)2(t-2)=0
Thus, the possible values of t are t=-1 or t=2.

Case 1: If t=2
Substituting t=2 into the system of equations gives:
α=β=γ
Since α2+β2+γ2=6, we have:
3α2=6α=±2
Thus, if α=2, then t=2.
Therefore, t+3=2+3=5.
This matches (S) → (5).

Case 2: If t=-1
Substituting t=-1 into the system equations, they all simplify to:
α+β+γ=0
If α=3, then:
β+γ=-3
Squaring both sides:
(β+γ)2=3
This matches (R) → (4).

Additionally, since u and w are perpendicular, their dot product is zero:
u·w=0α+β-2γ=0
Subtracting this from α+β+γ=0:
3γ=0γ=0
Thus, γ2=0.
This matches (Q) → (1).

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