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NCERT Class XII Mathematics Chapter - - Solutions

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Question 35 - 38: In the following cases, find the coordinates of the foot of theperpendicular drawn from the origin.
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Question : 35 of 78
Marks: +1, -0
2x + 3y + 4z – 12 = 0
Solution:  
Let the coordinates of the foot of the perpendicular P from the origin to the plane be (x1,y1,z1)(x_1, y_1, z_1) .
Then the direction ratios of the line OP are x1,y1,z1x_1, y_1, z_1. Writing the equation of the plane in the normal form, we have
229x+329y+429z=1229\frac{2}{\sqrt{29}}x+\frac{3}{\sqrt{29}}y+\frac{4}{\sqrt{29}}z=\frac{12}{\sqrt{29}}
where 229,329,429\frac{2}{\sqrt{29}},\frac{3}{\sqrt{29}},\frac{4}{\sqrt{29}} are the direction cosines of OP.
Since direction cosines and direction ratios of a line are proportional, we have x1229=y1329=z1429=k\frac{x_1}{\frac{2}{\sqrt{29}}}=\frac{y_1}{\frac{3}{\sqrt{29}}}=\frac{z_1}{\frac{4}{\sqrt{29}}}=k i.e., x1=2k29,y1=3k29x_1=\frac{2k}{\sqrt{29}},y_1=\frac{3k}{\sqrt{29}}, z1=4k29z_1=\frac{4k}{\sqrt{29}}
Substituting these values in the equation of plane, we get k=1229k=\frac{12}{\sqrt{29}}
Hence foot of perpendicular is (2429,3629,4829)\left(\frac{24}{29},\frac{36}{29},\frac{48}{29}\right)
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