Wondering if anyone could help?
1 answer:
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Starting from the formula
We can divide all the angles by 2 and we get
And finally
Angle x is a third quadrant angle, meaning that it is somewhere between 180 and 270. This implies that x/2 lies between 90 and 135. So, x/2 is a second quadrant angle, and its cosine is negative. This means that we can update the previous equation to
You know that
so, plug that value into the expression and you'll get cos(x/2).