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Trigonometry
Question

Angle $$\alpha$$ is in quadrant III and angle $$\beta$$ is in quadrant IV. If $$\sin \alpha = - \frac { 5 } { 6 }$$ and $$\cos \beta = \frac { 1 } { 2 }$$ , find $$\cos ( \alpha + \beta )$$ .

$$\frac { \sqrt { 11 } + 5 \sqrt { 3 } } { 6 }$$

$$\frac { \sqrt { 11 } 5 \sqrt { 3 } } { 6 }$$

$$\frac { \sqrt { 11 } 5 \sqrt { 3 } } { 12 }$$

$$\frac { \sqrt { 11 } | 5 \sqrt { 3 } } { 12 }$$