According to this law if law if two vector acting simultaneously at a point can be represented both in magnitude and direction by the two adjacent sides of a parallelogram, the resultant is represented completely (both in magnitude and direction) by the diagonal of the parallelogram passing through that point.
Suppose two vectors A and B inclined to each other at an angle θ be represented in magnitude and direction both by the concurrent sides vector PQ and PT of the parallelogram PQRT as shown in the fig.
According to the law of parallelogram resultant of vector A and B IS represented both in magnitude and direction by the diagonal vector PR of the parallelogram.
The magnitude of vector PR, i.e., |vector PR| can be given as,
Suppose the resultant vector R makes an angle α with the vector A, then
