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If 0 < (theta) and cos(theta)=0.5, then what is the value of cos(2theta)?

2 answers

First solve cos(theta)=0.5

Step 1: Where is the cosine of theta equal to 0.5? Think about your memory angles or special triangles.

Step 2: Now double the angles found in step 1, and take the cosine of the new angle you found. (Both answers should be the same).
Another approach using double angle identities.
cos(2A)=cos²(A)-sin²(A)=2cos²(A)-1
You will find cos(2A) by subtituting the value of cos(A) in the above equation.
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