If O is the origin and the line OP of length  r makes an angle α with X-axis and lies in the XY-plane then the coordinates of P are

OP lies in XY- plane and makes α angle with X-axis ⇒ it makes π2−α with Y- axis and π2 with Z- axis. d.c’s of OP are  (l,m,n)=cosα,cosπ2−α,cosπ2=(cosα,sinα,0),  P=(lr,mr,nr)