Q.

Find the locus of a point whose distance from x-axis is twice the distance from the point (1,-1,2):

1 y2+2x-2y-4z+6=0  
2 x2+2x-2y-4z+6=0  
3 x2-2x+2y-4z+6=0  
4 z2-2x+2y-4z+6=0  

Ans.

(3)

Let the point P(xp,yp,zp) be the required point,

the distance of the point from x-axis is yp2+zp2, The distance from the point (1,-1,2) is

⇒ yp2+zp2=(xp-1)2+(yp+1)2+(zp-2)2⇒xp2-2xp+2yp-4zp+6=0

Therefore, the locus of point P is  x2-2x+2y-4z+6=0