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Find the vector eq of the plane which passes through the points \( 2 \bar{i}+4 \bar{j}+2 \bar{k}, 2 \bar{i}+3 \bar{d}+5 \bar{k} \) and parallel to \( 3 \bar{i}-2 \bar{j}+\bar{k} \). Also find the point where this plane meets the joining the points \( 2 \bar{i}+\bar{j}+3 \bar{k} \) and \( 4 \bar{i}+2 \bar{j}+3 \bar{k} \)(Mar-2012) |
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Answer» Given that required plane passes through points P(2, 4, 2) and Q(2, 3, 5) and parallel to a line whose direction ratios are 3, -2, 1. Now, direction ratios of line AB are 0, -1, 3 Let the direction ratios of normal to the plane are A, B and C. Since, point P and Q lie on plane, therefore line joining points P and Q is perpendicular to normal. ∴ 0 x A + (-1) x B + 3 x C = 0 (condition of two perpendicular lines) ⇒ -B+3C = 0 ⇒ B=3C .....(i) Since, given plane is parallel to line 3i - 2j + k ∴ Normal to plane is perpendicular o line 3i - 2j + k ∴ 3A-2B+C = 0 ⇒ 3A-6C+C = 0 (from(i)) ⇒ 3A = 5C ⇒ A = 5C/3 .....(ii) Hence, the direction ratios of normal to required plane are 5C/3, 3C, C or 5, 9, 3. (By putting C=3) Therefore, the equation of the plane is 5x+9y+3z = d, where d is perpendicular distance of origin from plane. Since, point (2, 4, 2) lies on plane. ∴ d = 2 x 5 + 4 x 9 + 2 x 3 = 10+36+6 = 52 ∴ Equation of required plane is 5x+9y+3z = 52. Equation of line joining the points 2i+j+3k and 4i+2j+3k is r = a+λ(b-a) ⇒ r = (2i+j+3k) + λ((4-2)i + (2-1)j + (3-3)k ⇒ xi+yj+zk = (2+2λ)i + (1+λ)j + 3k (∵ r = xi+yj+zk) ⇒ x = 2+2λ, y = 1+λ, z = 3 Hence, locus of arbitrary point on line joining points (2, 1, 3) and (4, 2, 3) is given by. (x, y, z) = (2(1+2λ)), 1+λ, 3j Since, line meets the plane, therefore equation of arbitrary point on line satisfies equation of plane. ∴ 5 x 2(1+λ) + 9 x (1+λ) + 3 x 3 = 52 ⇒ 10+10λ+9+9λ+9 = 52 ⇒ 19λ = 52-28 = 24 ⇒ λ = 24/19 ∴ point which lies on both line and plane is (x, y, z) = (86/19, 43/19, 3). Hence, plane meets the line at (86/19, 43/19, 3). |
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