Trigonometry/More About Addition Formulas< Trigonometry
In the last row of expressions, if then the trigonometric functions are all positive so the positive sign is needed before the square root.
Using cofunctions we know that . Use the formula for and cofunctions we can write
Having derived we replace with and use the fact that cosine is even and sine is odd.
Using and the fact that cosine is even and sine is odd, we have