Cos 21 Degrees

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

What is cos 21°?

Answer

cos 21° ≈
0.9336

Angle in radians: 21° = 7π/60. The point of the angle is in quadrant I, where cosine is positive.

≈ — rounded to 4 decimal places. An exact form of cos 21° in square roots exists, but it is unwieldy — nested radicals.

P(cos 21°, sin 21°) ≈ (0.9336, 0.3584)

xyOP

P(cos 21°, sin 21°) ≈ (0.9336, 0.3584)

Highlighted: the projection on the x-axis — cosine, x ≈ 0.9336.

All four functions of 21°
FunctionExact valueDecimal
sin 21°—≈ 0.3584
cos 21°—≈ 0.9336
tan 21°—≈ 0.3839
cot 21°—≈ 2.6051

See Also

Recalculate with different values ↑

Cosine of nearby angles

Anglecos
18°√(10 + 2√5)/4
19°≈ 0.9455
20°≈ 0.9397
21°≈ 0.9336
22°≈ 0.9272
23°≈ 0.9205
24°≈ 0.9135

FAQ

What is cos 21°?

cos 21° ≈ 0.9336

Is there an exact value of cos 21°?

Yes: 21° 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 21° ≈ 0.9336.

Why does a calculator show −0.5477 for cos 21?

The calculator is in radian mode (RAD): it takes cos 21 as the cosine of 21 radians, cos 21 rad ≈ −0.5477. Switch it to degrees (DEG): cos 21° ≈ 0.9336. In radians the value is never undefined: π is irrational, so a whole number of radians never lands exactly on an axis.

See Also