Answer:
Step-by-step explanation:
The given trigonometric equation is .
We can either use the Pythagorean identity or the right angle triangle to solve for .
According to the Pythagorean identity,
Recall that, the cosine function is an even function, therefore
.
We substitute this value in to the above Pythagorean identity to get;
But we were given that,
, so we choose the negative value.
The correct answer is B