\overrightarrow{SA} = \overrightarrow{SR} + \overrightarrow{RA} = \frac{1}{2} \overrightarrow{PR} - \overrightarrow{b} = \frac{1}{2} \left( \overrightarrow{PA} + \overrightarrow{AR} \right) - \overrightarrow{b} = -\frac{1}{2} \overrightarrow{a} -\frac{1}{2} \overrightarrow{b}