$\begin {array} {1 1} (1)\;\large\frac{1}{3}( a\overrightarrow i - b \overrightarrow j) & \quad (2)\;\large\frac{2}{3} ( a \overrightarrow i + b \overrightarrow j) \\ (3)\;\large\frac{2}{3} ( a \overrightarrow i - b \overrightarrow j) & \quad (4)\;\large\frac{1}{3} ( a \overrightarrow i + b \overrightarrow j) \end {array}$