CBSE Class 12-science Answered
u have given 2-3 questions of the same type. so here's the method.
Assume that the perpendicular from the given point A to the given plane meets the plane at say pt P(a,b,c)
then P(a,b,c) is a point of the plane also. so it must satisfy the equation of the plane.
so we'll get
2a-b+c+3=0...(i)
since the perpendicular to the plane must be parallel to the normal to the plane
so the d.r.s of the line joining AP have to be 2,-1,1[since the plane has the eqn 2 x-1 y+1 z+3=0
let the reqd pt be Q then P is the mid point of AQ. as Q is the reflection of A in the plane and has to be lying along the line AP.
using this, you can find the coordiantes of P.
Take eqn of line AP in the form
a-1/1=b-2/2=c-3/3=k
find a,b,c in terms of k, substitute in eqn of plane.
once you have k,
take the point P as mid point of AQ and get coordinates of Q.