Use the Formula for the Cosine of the Difference of Two Angles
-$\cos \left( 155 ^ { \circ } \right) \cos \left( 35 ^ { \circ } \right) + \sin \left( 155 ^ { \circ } \right) \sin \left( 35 ^ { \circ } \right)$
A) $- \frac { 1 } { 2 }$
B) $- \frac { \sqrt { 3 } } { 2 }$
C) $- \sqrt { 3 }$
D) $- 2$

Question

Quizplus · Accepted Answer

The given expression is an application of the cosine difference identity, $\cos(A - B) = \cos A \cos B + \sin A \sin B$. Substituting $A = 155^\circ$ and $B = 35^\circ$, we get $\cos(155^\circ - 35^\circ) = \cos(120^\circ)$, which equals $-\frac{1}{2}$.