Cos 9 Degrees

Sin, Cos, Tan, Cot of an Angle
Function
Angle (°)

What is cos 9°?

Answer

cos 9° ≈
0.9877

Angle in radians: 9° = π/20. The point of the angle is in quadrant I, where cosine is positive.

≈ — rounded to 4 decimal places. An exact form of cos 9° in square roots exists, but it is unwieldy — nested radicals.

P(cos 9°, sin 9°) ≈ (0.9877, 0.1564)

xyOP

P(cos 9°, sin 9°) ≈ (0.9877, 0.1564)

Highlighted: the projection on the x-axis — cosine, x ≈ 0.9877.

All four functions of 9°
FunctionExact valueDecimal
sin 9°—≈ 0.1564
cos 9°—≈ 0.9877
tan 9°—≈ 0.1584
cot 9°—≈ 6.3138

See Also

Recalculate with different values ↑

Cosine of nearby angles

Anglecos
6°≈ 0.9945
7°≈ 0.9925
8°≈ 0.9903
9°≈ 0.9877
10°≈ 0.9848
11°≈ 0.9816
12°≈ 0.9781

FAQ

What is cos 9°?

cos 9° ≈ 0.9877

Is there an exact value of cos 9°?

Yes: 9° 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 9° ≈ 0.9877.

Why does a calculator show −0.9111 for cos 9?

The calculator is in radian mode (RAD): it takes cos 9 as the cosine of 9 radians, cos 9 rad ≈ −0.9111. Switch it to degrees (DEG): cos 9° ≈ 0.9877. In radians the value is never undefined: π is irrational, so a whole number of radians never lands exactly on an axis.

See Also