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If the two lines given by

x=py+q,z=ry+sx=py+q, z=ry+s and

x=p′y+q′,z=r′y+s′x=p'y+q', z=r'y+s'

are perpendicular, then which of the following is equal to -1 ?

Solution

✅ Correct Option: 4

Treat yy as the parameter. Then the first line has direction ratios (p,1,r)(p,1,r) and the second has (p′,1,r′)(p',1,r').

Perpendicularity requires the dot product to vanish: pp′+1+rr′=0pp' + 1 + rr' = 0.

Hence pp′+rr′=−1pp' + rr' = -1.

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