Q28 / 54 · JEE Advanced 2020 (Paper 2) · Mathematics

Let α, β, γ, δ be real numbers such that α2+β2+γ2≠0 and α+γ=1. Suppose the point 3,2,-1 is the mirror image of the point 1,0,-1 with respect to the plane αx+βy+γz=δ. Then which of the following statements is/are TRUE?

  • Correct answer
    A
    α+β=2
  • Correct answer
    B
    δ-γ=3
  • Correct answer
    C
    δ+β=4
  • D
    α+β+γ=δ

Solution

Figure

From the diagram,

Mid point of A & A' lies on the plane, i.e.,  B2,1,-1

2α+β-γ=δ…1

α+γ=1  ….2

Direction Ratio of AA'2,2,0 will be proportional to normal to the plane,

α2=β2=γ0=λ

α=2λ, β=2λ, γ=0

∴2λ+0=1 ⇒λ=12

∴α=1, β=1, γ=0, δ=3

Now,

(A) α+β+γ=1+1+0=2≠δ

(B) α+β=1+1=2
(C) δ+β=3+1=4
(D) δ-γ=3

Hence, the correct answer is options: 'A', 'B', 'C'.

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