Use the Sum-To-Product Formulas to Find the Exact Value of the Given

Multiple Choice

Use the sum-to-product formulas to find the exact value of the given expression. ​​ $$\cos 150 ^ { \circ } + \cos 30 ^ { \circ }$$ ​

A) ​ $\frac { - \sqrt { 3 } } { 2 }$
B) ​1
C) ​ $\frac { \sqrt { 3 } } { 2 }$
D) -1
E) 0