We can only use the sine and cosine functions of `30^"o"`, so we need to start with `60^"o" = 2 × 30^"o"`.
We will use the result
cos 2α = cos2 α − sin2 α,
and the 30-60 triangle:
Now we proceed to find the exact value of cos60° using the ratios of 30°:
These exercises are really here for practice on the double angle formula. Of course, we could have found the value of cos60° directly from the triangle.
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