The cosine double angle formula is cos(2theta)=cos2(theta) - sin2(theta). Combining this formula with the Pythagorean Identity, cos2(theta) + sin2(theta)=1, two other forms appear: cos(2theta)=2cos2(theta)-1 and cos(2theta)=1-2sin2(theta). These can be used to find the power-reduction formulas, which reduce a second degree or higher trig function to a first degree. These formulas are very useful in Calculus.
Sample Problems (2)
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