Trigonometric Identities I

Trigonometric identities are equations that relate trigonometric functions and hold for all angles within their domains. A fundamental identity among these is the Pythagorean identity, which arises directly from the geometry of the unit circle. For any angle θ, a point on the unit circle has coordinates (cos⁡ θ, sin ⁡θ), and since the radius of the circle is one, the Pythagorean Theorem gives:

This identity serves as the basis for deriving additional identities. Dividing the Pythagorean identity by cos⁡²θ (assuming cos ⁡θ ≠ 0) yields:

The Pythagorean identity also plays a role in more complex identities, such as the cosine addition formula. One method of deriving this formula involves geometric arguments using points on the unit circle and the distance formula. In this derivation, the coordinates of key points involve combinations of sine and cosine values. When applying the distance formula and simplifying, the Pythagorean identity reduces expressions involving the squares of trigonometric terms. This ultimately leads to:

By replacing t with -t in the addition formula and using the even and odd properties of cosine and sine, the subtraction formula is obtained:

The sine addition and subtraction formulas can also be derived using cofunction identities and angle transformations:

Finally, the tangent addition and subtraction identities are derived from the sine and cosine sum formulas by taking their ratio:

These identities are essential tools in algebraic simplification, solving trigonometric equations, and calculus applications