If all the points of a straight joining any two points on the surface lie on the surface then the surfae is called a plane.
Let the first degree equation in x, y, z be ax + by + cz + d = 0 ......... ( 1 )
Let A ( x1, y1, z1, ) and
B ( x2, y2, z2 ) be any two points in surface represented by ( 1 )
then ax1 + by1 + cz1 + d = 0 .......( 2 )
ax2 + by2 + cz2 + d = 0 ......( 3 )
Let be any point on 
Let C divide in the ratio m : n
