How to derive the
Start out with an formula that includes both cosine and sine
Solve for
First add
Now divide each side by 2 and you will get the sine power-reducing formula.
Half-Angle Formulas
How to derive the sine half-angle formula:
This allows you to substitute u into the equation for theta.
Now your formula should look like:
Take the square root of each side. Remember that the square root of the right side will be
The formula should now look like this:
Your final answer can either be + or - but it can not be both! The sign of the final answer depends where it is located on the unit circle.
If you wanted to find
Sum-to-Product Formulas
Product-to-Sum Formulas
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