Identités trigonométriques I

The Pythagorean identity states that sine squared plus cosine squared equals one for all real angles. It is derived using the unit circle: any point on the circle has coordinates (cos θ, sin θ). Since the radius equals one, the Pythagorean Theorem applied to the right triangle formed by these coordinates proves this fundamental identity.

On the unit circle, any angle measured from the positive x-axis corresponds to a point whose x-coordinate is the cosine and y-coordinate is the sine of that angle. This geometric relationship connects trigonometric functions of real numbers to circular coordinates, forming the foundation for deriving identities and understanding angle behavior.

The sine and cosine addition formulas express sin(A + B) and cos(A + B) in terms of individual sine and cosine values. These formulas are derived using geometric arguments with unit circle points and the distance formula. The Pythagorean identity simplifies the resulting expressions to yield the final addition formulas.

The tangent addition formula is derived by taking the ratio of the sine and cosine addition formulas. Since tangent equals sine divided by cosine, dividing the sine addition formula by the cosine addition formula produces the tangent addition identity for combined angles.

In astronomy, celestial coordinates describe star positions on the celestial sphere using the Pythagorean identity. The identity ensures these positions remain consistent on the sphere, allowing astronomers to accurately track stars as they appear to move across the sky in three-dimensional space.

The sine subtraction formula is obtained by replacing the angle variable with its negative in the sine addition formula, then applying the even and odd properties of cosine and sine. This transformation converts sin(A + B) into sin(A - B) while maintaining the identity's validity.

Trigonometric identities are essential tools for algebraic simplification and solving trigonometric equations. By applying identities like the Pythagorean identity and addition formulas, complex trigonometric expressions can be reduced to simpler forms, making equations easier to solve and enabling calculus applications.