三角関数の恒等式 III

Cofunction identities describe how pairs of trigonometric functions relate when their angles are complementary, meaning they add up to 90 degrees. These identities reveal that sine and cosine interchange roles for complementary angles, and all other trigonometric functions inherit this property. They provide flexibility in real-world calculations, such as measuring distances from moving boats to lighthouses.

On the unit circle, a point at angle θ from the positive x-axis has coordinates (cos θ, sin θ). The same point measured from the positive y-axis at angle 90° − θ has coordinates (sin(90° − θ), cos(90° − θ)). Since both descriptions refer to the same point, equating these expressions proves the basic cofunction identities for sine and cosine.

Complementary angles are two angles that sum to 90 degrees. When angles are complementary, their trigonometric functions exhibit a special symmetry: sine of one angle equals cosine of the other, and tangent of one equals cotangent of the other. This relationship demonstrates the deep connection between complementary angles in right triangles and the unit circle.

Since all trigonometric functions are based on sine and cosine, they inherit the cofunction property. Tangent and cotangent, secant and cosecant all follow the same complementary angle relationship. For example, tangent of θ equals cotangent of (90° − θ), allowing flexible substitutions in calculations while preserving results.

Cofunction identities enable practical problem-solving in navigation. When measuring distance from a moving boat to a lighthouse using the angle of elevation, cofunction identities allow replacing tangent of θ with cotangent of the complementary angle in the distance formula. This substitution preserves accuracy while providing computational flexibility.

On the unit circle, measuring angle θ from the positive x-axis gives coordinates (cos θ, sin θ). When measuring the same point from the positive y-axis using angle 90° − θ, the horizontal segment becomes opposite and the vertical segment becomes adjacent. This geometric transformation causes sine and cosine to swap positions, establishing the cofunction relationship.

Cofunction identities simplify trigonometric problem solving by allowing angle transformations and function substitutions. They connect complementary angles systematically, enabling students to work with trigonometric functions problem solving more efficiently. These identities are foundational for understanding how different angle measurements produce equivalent results.