Answer:
-⅗
Explanation:
According to the Unit Circle, tan 0,6435011088 is ¾, so we plug this into the cosine function to get this:
cos 0,6435011088 - 1½π >> ¾
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*** Plus, we need to make sure that the x-value falls in between 0 and π\2, which it did, so no need to worry.
**** 1½π = 3⁄2π
