Cos 81 Degrees
Sin, Cos, Tan, Cot of an Angle
What is cos 81°?
Answer
cos 81° ≈
0.1564
Angle in radians: 81° = 9π/20. The point of the angle is in quadrant I, where cosine is positive.
≈ — rounded to 4 decimal places. An exact form of cos 81° in square roots exists, but it is unwieldy — nested radicals.
P(cos 81°, sin 81°) ≈ (0.1564, 0.9877)
P(cos 81°, sin 81°) ≈ (0.1564, 0.9877)
Highlighted: the projection on the x-axis — cosine, x ≈ 0.1564.
See Also
- Arcsin, Arccos, Arctan, Arccot - The angle from the value of its sine, cosine, tangent or cotangent: exact in degrees and π, with the check and the unit circle
- Square Root Calculator - Calculate the square root of any number
- Simplify Square Root Calculator - Write any square root in simplest radical form, step by step
- Geometry Formulas: Area, Volume, Perimeter - The formula for every standard shape, where it comes from, and worked examples
Cosine of nearby angles
FAQ
What is cos 81°?
cos 81° ≈ 0.1564
Is there an exact value of cos 81°?
Yes: 81° is a multiple of 3°, and the values of such angles can be written with square roots — but the expression is unwieldy, with nested radicals built from the values of 18° and 15° (3° = 18° − 15°). In practice the decimal is used: cos 81° ≈ 0.1564.
Why does a calculator show 0.7767 for cos 81?
The calculator is in radian mode (RAD): it takes cos 81 as the cosine of 81 radians, cos 81 rad ≈ 0.7767. Switch it to degrees (DEG): cos 81° ≈ 0.1564. In radians the value is never undefined: π is irrational, so a whole number of radians never lands exactly on an axis.
See Also
- Cos 71 Degrees: Cosine of 71° on the Unit Circle
- Cos 72 Degrees: Cosine of 72° on the Unit Circle
- Cos 73 Degrees: Cosine of 73° on the Unit Circle
- Cos 74 Degrees: Cosine of 74° on the Unit Circle
- Cos 75 Degrees: Cosine of 75° on the Unit Circle
- Cos 76 Degrees: Cosine of 76° on the Unit Circle
- Cos 77 Degrees: Cosine of 77° on the Unit Circle
- Cos 78 Degrees: Cosine of 78° on the Unit Circle
- Cos 79 Degrees: Cosine of 79° on the Unit Circle
- Cos 80 Degrees: Cosine of 80° on the Unit Circle
- Cos 82 Degrees: Cosine of 82° on the Unit Circle
- Cos 83 Degrees: Cosine of 83° on the Unit Circle
- Cos 84 Degrees: Cosine of 84° on the Unit Circle
- Cos 85 Degrees: Cosine of 85° on the Unit Circle
- Cos 86 Degrees: Cosine of 86° on the Unit Circle
- Cos 87 Degrees: Cosine of 87° on the Unit Circle
- Cos 88 Degrees: Cosine of 88° on the Unit Circle
- Cos 89 Degrees: Cosine of 89° on the Unit Circle
- Cos 90 Degrees: Cosine of 90° on the Unit Circle
- Cos 97 Degrees: Cosine of 97° on the Unit Circle
- Косинус 81 градуса (cos 81°) на единичной окружности
- Coseno de 81 grados (cos 81°) en el círculo unitario
- Cosseno de 81 graus (cos 81°) no ciclo trigonométrico