Find the coordinates of a point on y-axis which are at a distance of 5√2 from the point P(3, −2, 5).
Let Q(0, y, 0) be any point on y-axis.
Then
PQ=√(0−3)2+(y+2)2+(0−5)2
= √9+y2+4+4y+25
= √y2+4y+38
But √y2+4y+38=5√2
Squaring both sides, we have
y2+4y+38=50 ⇒ y2+4y−12
=0 ⇒ (y−2)(y+6)=0
⇒ y=2,−6
Thus coordinates of point Q are (0, 2, 0) and (0, −6, 0).