Use the Formula for the Cosine of the Difference of Two

Multiple Choice

Use the Formula for the Cosine of the Difference of Two Angles
- $\cos \left( \frac { 7 \pi } { 18 } \right) \cos \left( \frac { 2 \pi } { 9 } \right) + \sin \left( \frac { 7 \pi } { 18 } \right) \sin \left( \frac { 2 \pi } { 9 } \right)$

A) $\alpha = \frac { 7 \pi } { 18 } , \beta = \frac { 2 \pi } { 9 }$
B) $\alpha = \frac { 7 \pi } { 18 } , \beta = - \frac { 2 \pi } { 9 }$
C) $\alpha = - \frac { 2 \pi } { 9 } , \beta = \frac { 7 \pi } { 18 }$
D) $\alpha = \frac { 2 \pi } { 9 } , \beta = \frac { 7 \pi } { 18 }$ Write the expression as the cosine of an angle, knowing that the expression is the right side of the formula for $$\cos ( \alpha - \beta ) \text { with particular values for } \alpha \text { and } \beta .$$