cos21

Cos21 is a shorthand term that generally refers to the cosine of the angle 21 degrees, written as cos(21°) in trigonometry. When the angle is measured in degrees, cos(21°) evaluates to a value close to 0.93358. If the angle were instead specified in radians, cos(21) would denote the cosine of 21 radians, a different quantity that depends on the chosen unit and typically requires a calculator in radian mode.

Exact expressions for cos(21°) do not simplify to a simple radical or surd. While trigonometric identities can be used to derive algebraic expressions for certain angle multiples, cos(21°) does not have a compact closed form. In practice, it is computed numerically to high precision using series expansions, iterative methods, or standard calculators and software.

Cosine values at specific angles such as 21° frequently appear in geometry, physics, engineering, and computer science. They are used in solving triangles, performing angle-based analyses, evaluating trigonometric identities, and in signal processing tasks where precise angle measurements are involved. The cosine function is periodic and even, with cos(−θ) = cos(θ) and cos(θ + 360°) = cos(θ).

Notes on interpretation: because the cosine function requires a unit for the angle, cos21 can be ambiguous without a degree or radian symbol. In typical mathematical writing, cos(21°) explicitly denotes degrees, while cos(21) indicates radians unless otherwise stated.