Use the angle sum or difference identity to find the exact value of
cos195
You have to figure out a way to make 195° using
the sum or difference of two special angles from this list:
30°, 45°, 60°, 90°, 120°, 135°, 150°, 180°, 210°,
225°, 240°, 270°, 300°, 315°, 330°.
You can write 195° as any one of these:
150°+45°, 135°+60°, 225°-30°, 240°-45°, 315°-120°, 330°-135°
If you choose to use the first one, 150°+45°, you then use
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Let's convert that to degrees:
°
Then we can write 75° as 30°+45°
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Then we can use the identity: 
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Rationalize the denominator:
Indicate multiplication of numerators and denominators:
Multiply out the bottom:
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Move the negative sign out front, and cancel the 4's
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Edwin
