Find the Exact Value of the Expression Using the Provided $\cos ( s + t )$

question 28

Multiple Choice

Find the exact value of the expression using the provided information.
-Find $\cos ( s + t )$ given that $\sin s = - \frac { \sqrt { 3 } } { 3 }$ , with $s$ in quadrant IV, and $\sin t = - \frac { \sqrt { 5 } } { 6 }$ , with $t$ in quadrant IV.

• A) \(\frac { \sqrt { 186 } - \sqrt { 15 } } { 18 }\)
• B) \(\frac { \sqrt { 55 } + \sqrt { 15 } } { 18 }\)
• C) \(\frac { \sqrt { 55 } - \sqrt { 15 } } { 18 }\)
• D) \(\frac { \sqrt { 186 } + \sqrt { 15 } } { 18 }\)