Question

Explain: When an angle between two vectors increases the magnitude of their resultant decreases

Answer 1

The reason for, why the resultant decreases with the increase in the angle between $2$ vectors can be explained with an example:

You and I pull with a northward force of $200N$ on a $10kg$ box.

The resultant force $=200+200=400N$ North

You pull with a northward force of $200N$ on one end of the box, and I pull with a southward force of $200N$ on the other end of the box.
The resultant force $=+200+-200=0N$
The box does not move, because the $2$ forces are in opposite directions.