Thursday, April 5, 2012

Power-Reducing Formulas, Half-Angle Formulas, Sum-to-Product Formulas, Product-to-Sum Formulas

Power-Reducing Formulas

How to derive the  power-reducing formula:


Start out with an formula that includes both cosine and sine



Solve for 


First add  to each side. This will cancel it on the right side of the formula. Then you must subtract  from each side. Your formula should now look like this: 





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   you would have to use  in the half-angle formula.


Sum-to-Product Formulas
















Product-to-Sum Formulas



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