Trigonometry/More About Addition Formulas
Related Formulae edit
Tangent Formulae edit
In the last row of expressions, if then the trigonometric functions are all positive so the positive sign is needed before the square root.
Derivations edit
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.
Related Formulae edit
Derivations edit
Using and the fact that cosine is even and sine is odd, we have