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JEE Advanced 2015 · Introduction to Three Dimensional Geometry

In R3R^{3}, consider the planes P1:y=0P_{1}: y = 0 and P2:x+z=1P_{2}: x + z = 1. Let P3P_{3} be the plane, different from P1P_{1} and P2P_{2}, which passes through the intersection of P1P_{1} and P2P_{2}. If the distance of the point (0,1,0)(0, 1, 0) from P3P_{3} is 11 and the distance of a point (α,β,γ)(\alpha, \beta, \gamma) from P3P_{3} is 22, then which of the following relations is (are) true?

Answers: B. 2α−β+2γ+4=02\alpha - \beta + 2\gamma + 4 = 0 · D. 2α−β+2γ−8=02\alpha - \beta + 2\gamma - 8 = 0

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In R³, consider the planes P₁: y = 0 and P₂: x + z = 1. Let P₃ be the plane, different from P₁ and P₂, which passes through the intersection of P₁ and P₂. If the distance of the point (0, 1, 0) from P₃ is 1 and the distance of a point (α, β, γ) from P₃ is 2, then which of the following relations is (are) true? P1: y=0,P2: x+z=1P_1:\ y = 0, \qquad P_2:\ x + z = 1 P3 passes through P1∩P2,P3≠P1,P2P_3 \text{ passes through } P_1 \cap P_2, \quad P_3 \neq P_1, P_2

dist((0,1,0),P3)=1,dist((α,β,γ),P3)=2\text{dist}\big((0,1,0), P_3\big) = 1, \qquad \text{dist}\big((\alpha,\beta,\gamma), P_3\big) = 2

Find P3, then test which of (1)–(4) hold\text{Find } P_3, \text{ then test which of (1)–(4) hold} P1: y=0 (xz-plane),P2: x+z=1P_1:\ y = 0 \ (\text{xz-plane}), \qquad P_2:\ x + z = 1 P1∩P2: y=0, x+z=1(a line)P_1 \cap P_2:\ y = 0,\ x + z = 1 \quad (\text{a line}) P3: (x+z−1)+k y=0P_3:\ (x + z - 1) + k\,y = 0 ⇒ x+ky+z−1=0,n⃗=(1, k, 1)\Rightarrow\ x + ky + z - 1 = 0, \qquad \vec n = (1,\ k,\ 1) ∣0+k(1)+0−1∣1+k2+1=1\dfrac{|0 + k(1) + 0 - 1|}{\sqrt{1 + k^2 + 1}} = 1 ∣k−1∣k2+2=1\dfrac{|k - 1|}{\sqrt{k^2 + 2}} = 1 (k−1)2=k2+2(k - 1)^2 = k^2 + 2 k2−2k+1=k2+2⇒−2k=1⇒k=−12k^2 - 2k + 1 = k^2 + 2 \Rightarrow -2k = 1 \Rightarrow k = -\dfrac{1}{2} P3: x−12y+z−1=0P_3:\ x - \tfrac{1}{2}y + z - 1 = 0 2x−y+2z−2=0,∣n⃗∣=4+1+4=32x - y + 2z - 2 = 0, \qquad |\vec n| = \sqrt{4 + 1 + 4} = 3 ∣2α−β+2γ−2∣3=2\dfrac{|2\alpha - \beta + 2\gamma - 2|}{3} = 2 ∣2α−β+2γ−2∣=6|2\alpha - \beta + 2\gamma - 2| = 6 2α−β+2γ−2=6⇒2α−β+2γ−8=0 (4)2\alpha - \beta + 2\gamma - 2 = 6 \Rightarrow 2\alpha - \beta + 2\gamma - 8 = 0 \ \text{(4)} 2α−β+2γ−2=−6⇒2α−β+2γ+4=0 (2)2\alpha - \beta + 2\gamma - 2 = -6 \Rightarrow 2\alpha - \beta + 2\gamma + 4 = 0 \ \text{(2)} True relations: (2) and (4)\boxed{\text{True relations: } (2) \text{ and } (4)} (1),(3): β sign and γ coefficient do not match(1), (3): \ \beta \text{ sign and } \gamma \text{ coefficient do not match} P3: 2x−y+2z−2=0,true: (2),(4)P_3:\ 2x - y + 2z - 2 = 0, \quad \text{true: } (2), (4)

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