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3D Geometry question

2020 · 9 Jan · Shift 1 · Q30
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3D Geometry question

2020 · 9 Jan · Shift 1 · Q30

JEE MainMathematics3D GeometryNumerical+4 / −1
The projection of the line segment joining the points (1, –1, 3) and (2, –4, 11) on the line joining the points (–1, 2, 3) and (3, –2, 10) is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 8

  1. Let the given points be

A(1,−1,3),B(2,−4,11)A(1,-1,3), \quad B(2,-4,11)A(1,−1,3),B(2,−4,11)

and

C(−1,2,3),D(3,−2,10).C(-1,2,3), \quad D(3,-2,10).C(−1,2,3),D(3,−2,10).

We need the projection of the line segment ABABAB on the line CDCDCD.

  1. Find the direction vectors:

For ABABAB, AB⃗=B−A=(2−1, −4−(−1), 11−3)=(1,−3,8).\vec{AB}=B-A=(2-1,\,-4-(-1),\,11-3)=(1,-3,8).AB=B−A=(2−1,−4−(−1),11−3)=(1,−3,8).

For CDCDCD, CD⃗=D−C=(3−(−1), −2−2, 10−3)=(4,−4,7).\vec{CD}=D-C=(3-(-1),\,-2-2,\,10-3)=(4,-4,7).CD=D−C=(3−(−1),−2−2,10−3)=(4,−4,7).

  1. The projection of vector AB⃗\vec{AB}AB on the line in the direction of CD⃗\vec{CD}CD is the scalar projection:

Projection=AB⃗⋅CD⃗∣CD⃗∣.\text{Projection} = \frac{\vec{AB}\cdot \vec{CD}}{|\vec{CD}|}.Projection=∣CD∣AB⋅CD​.

  1. Compute the dot product:

AB⃗⋅CD⃗=(1)(4)+(−3)(−4)+(8)(7)=4+12+56=72.\vec{AB}\cdot \vec{CD} = (1)(4)+(-3)(-4)+(8)(7)=4+12+56=72.AB⋅CD=(1)(4)+(−3)(−4)+(8)(7)=4+12+56=72.

  1. Compute the magnitude of CD⃗\vec{CD}CD:

∣CD⃗∣=42+(−4)2+72=16+16+49=81=9.|\vec{CD}|=\sqrt{4^2+(-4)^2+7^2} = \sqrt{16+16+49} = \sqrt{81}=9.∣CD∣=42+(−4)2+72​=16+16+49​=81​=9.

  1. Therefore,

Projection=729=8.\text{Projection} = \frac{72}{9}=8.Projection=972​=8.

So, the required projection is

8.\boxed{8}.8​.

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