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Question

Find the perpendicular distance from A(1,0,3) to the line r=(4,7,1)+k(1,2,2),kR. Also find the foot of the perpendicular.

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Solution

Consider
r=(4^i+7^j+^k)+k(1^i+2^j2^k)

Cartesian equation of the line

x41=y72=z12

Let,

x41=y72=z12=r

Then,

x=r+4,y=2r+7,z=2r+1

General point on the given line is
(r+4,2r+7,2r+1)

Let the foot of perpendicular to be,

P(r1+4,2r1+7,2r1+1)

Now,
Direction ratios of the line AP are

r1+41,2r1+70,2r1+13
that is

r1+3,2r1+7,2r22

Since AP is perpendicular to the given line

Therefore,

1(r1+3)+2(2r1+7)2(2r12)=0r1=73

Then,

r1+4=532r1+7=732r1+1=173

Therefore, foot of perpendicular is P(53,73,173)

And perpendicular distance =AP

=(531)2+(73)2+(1733)2=(23)2+(73)2+(83)2=4+49+643=1173

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