Component of vector

We know that magnitude of vect A + vect B is given by
√A²+B²+2ABcosθ
.
If A and B are perpendicular to each other, then the third term in the expression will become 0.
So we can say that Two perpendicular vectors contribute to the resultant independently.
If look at it from the point of view of resultant vector( say vect C), then we can say that it is composed oftwo vectors which are perpendicular to each other.

Thus when we say that we want to find x and y components of some vector D, we actually want to find the two perpendicular vectors whose resultant is D