Posted by **Parker** on Friday, August 14, 2009 at 2:46pm.

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

- Math (Find the Value) -
**Marth**, Friday, August 14, 2009 at 4:09pm
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).

- Math (Find the Value) -
**MathMate**, Friday, August 14, 2009 at 9:01pm
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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