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NDA 2016
Exam
NDA
Year
2016
Subject
Mathematics
Chapter
Three Dimensional Geometry

NDA 2016 · Mathematics · Three Dimensional Geometry

Paragraph: Let Q be the image of the point P(-2,1,-5) in the plane...

Paragraph: Let Q be the image of the point P(−2,1,−5)\mathrm{P}(-2,1,-5) in the plane 3x−2y+2z+1=03 x-2 y+2 z+1=0.
Question: Consider the following: 1. The coordinates of QQ are (4,−3,−1)(4,-3,-1). 2. PQ is of length more than 8 units. 3. The point (1,−1,−3)(1,-1,-3) is the mid-point of the line segment PQ and lies on the given plane. Which of the above statements are correct?

Options

  1. A1 and 2 only
  2. B2 and 3 only
  3. C1 and 3 only
  4. D1,2 and 3

Correct answer

D. 1,2 and 3

Step-by-step solution

Let Q(x1,y1,z1)Q \left(x_1, y_1, z_1\right) be the image of the point PP. The direction ratios of PQP Q are 3,−2,23,-2,2....(i) The Equation of line PQP Q is x+23=y−1−2=z+52=\frac{x+2}{3}=\frac{y-1}{-2}=\frac{z+5}{2}= rCoordinatesofanypointonthelineCoordinates of any point on the lineP Qis3r−2,−2r+1and2r−5is 3 r-2,-2 r+ 1 and 2 r-5. Let Q(3r−2,−2r+1,2r−5)Q(3 r-2,-2 r+1,2 r-5) be such a point. Let LL be the mid point of PQ,L=(3r2−2,1−r,r−5)P Q, L= \left(\frac{3 r}{2}-2,1-r, r-5\right) Since LL lies on the plane 3x−2y+2z+1=03 x-2 y+2 z+1=0 So, 3(3r2−2)3\left(\frac{3 r}{2}-2\right)-2(1-r)+2(r-5)+1=0\Rightarrow 172\frac{17}{2} r-17=0 \Rightarrow r=2So,coordinatesofSo, coordinates ofQareare(3 \times2-2,-2 \times2+1,2 \times2-5)=(4,-3,-1)...(ii)Alsothemidpointof...(ii) Also the mid point ofP QisisL=\left(\frac{3 \times 2}{2}-2,1-2,2-5\right)=(1,−1,−3)(1,-1,-3)...(iii) ∴PQ=(−2−4)2+(1+3)2+(−5+1)2=68⇒PQ=217>8\begin{aligned} & \therefore P Q=\sqrt{(-2-4)^2+(1+3)^2+(-5+1)^2}=\sqrt{68}\\ & \Rightarrow P Q=2 \sqrt{17} >8 \end{aligned}\therefore$ Option (4) is correct.
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