Refer to Fig. 1-19. In terms of vectors $\overrightarrow{\mathbf{A}}$ and $\overrightarrow{\mathbf{B}}$, express the vectors $(a) \overrightarrow{\mathbf{P}},(b) \overrightarrow{\mathbf{R}},(c) \overrightarrow{\mathbf{S}}$, and $(d) \overrightarrow{\mathbf{Q}}$.