A vector can be reported in two way:
(i) Magnitude and angle form : In this form vector is described in terms of its magnitude and angle from known directions (usually , & axes)
(ii) Component form : In this form, vector is described in terms of its & components.

There must be an interrelation between two of reporting same thing i.e. vector and is given as follows :
If a vector is represent in component form as then magnitude of vector is given by
and angle with axes respectively is given by
and Let a vector is written as then
(i) Magnitude of vector is-