evaluate cos150 degrees
Using a calculator or unit circle, we can find that cos(150 degrees) is equal to -0.866 or approximately -0.87 (rounded to two decimal places).
150° falls into the "exact" trig ratio category:
by the CAST rule:
cos 150
= - cos 30°
= - √3/2
Yes, you are correct! Thank you for clarifying.
Using the CAST rule, we know that 150 degrees is in the second quadrant, where cosine is negative.
Cosine of 30 degrees is a commonly memorized exact value of √3/2. Therefore, we can use the cosine identity cos(-θ) = cos(θ) to rewrite cos(150 degrees) as -cos(30 degrees), giving us:
cos(150 degrees) = -cos(30 degrees) = -√3/2
So, the exact value of cos(150 degrees) is -√3/2.