\(\overrightarrow R = \overrightarrow A + \overrightarrow B \)
\(|\overrightarrow R |\, = \,|\overrightarrow A + \overrightarrow B |\, = \sqrt {{A^2} + {B^2} + 2AB\cos \theta } \)
If \(\theta = 0^\circ \) then \(|\overrightarrow R |\, = A + B\)\( = \,|\overrightarrow A | + |\overrightarrow B |\)