define the resultant or composition of coplanar?

Answer 1

The resultant, or vector sum, of coplanar vectors is the vector that results when you first arrange all the vectors so that the tail of each successive vector touches the arrow of the previous vector, and then draw a vector from the tail of the first vector to the arrow of the last.

In this picture:

http://www.codesampler.com/d3dbook/chapter_02/chapter_02_files/image012.jpg

a + b is the resultant vector.

The resultant can also be found mathematically by separately summing all the x-components and y-components of all the vectors.

Answer 2

Coplanar means all points, lines, forces, etc are in the same plane. For example, the resultant of several vector forces lieing in the same plane would lie in the same plane.