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Question

A variable plane at a distance of 1 unit from the origin cuts the axes at A,B and C. If centroid D(x,y,z) of ΔABC satisfies the relation 1x2+1y2+1z2=k, then the value of k is

A
3
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B
1
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C
13
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D
9
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Solution

The correct option is A 9
Equation of plane at a distance 1 from origin is given by,
lx+my+nz=1, where l,m,n are direction cosine of normal to the plane along distance
Thus intercept on the axes are,
A=(1l,0,0),B=(0,1m,0),C=(0,0,1n)
So centroid of ABC is,
x=13l,y=13m,z=13n
Also we know,
l2+m2+n2=1
19x2+19y2+19z2=1
1x2+1y2+1z2=9
Hence, required value of k is 9.

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