Cos 255 Degrees

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

What is cos 255°?

Answer

cos 255° =
−(√6 − √2)/4 ≈ −0.2588

Angle in radians: 255° = 17π/12. The point of the angle is in quadrant III, where cosine is negative.

≈ — rounded to 4 decimal places.

cos 255° = cos(180° + 75°) = −cos 75° = −(√6 − √2)/4

P(cos 255°, sin 255°) = (−(√6 − √2)/4, −(√6 + √2)/4)

cos 75° = cos(45° + 30°) = (√2/2)(√3/2) − (√2/2)(1/2) = (√6 − √2)/4

xyOP

P(cos 255°, sin 255°) ≈ (−0.2588, −0.9659)

Highlighted: the projection on the x-axis — cosine, x ≈ −0.2588.

All four functions of 255°
FunctionExact valueDecimal
sin 255°−(√6 + √2)/4≈ −0.9659
cos 255°−(√6 − √2)/4≈ −0.2588
tan 255°2 + √3≈ 3.7321
cot 255°2 − √3≈ 0.2679

Reference angle: cos 75° = (√6 − √2)/4

See Also

Recalculate with different values ↑

Cosine of nearby angles

Anglecos
234°−√(10 − 2√5)/4
240°−1/2
252°−(√5 − 1)/4
255°−(√6 − √2)/4
270°0
285°(√6 − √2)/4
288°(√5 − 1)/4

FAQ

What is cos 255°?

cos 255° = −(√6 − √2)/4 ≈ −0.2588

How are sin 15° and cos 75° related?

They are equal: sin α = cos(90° − α), and 15° + 75° = 90°. So sin 15° = cos 75° = (√6 − √2)/4, cos 15° = sin 75° = (√6 + √2)/4, and tan 15° = cot 75° = 2 − √3.

Why does a calculator show −0.8623 for cos 255?

The calculator is in radian mode (RAD): it takes cos 255 as the cosine of 255 radians, cos 255 rad ≈ −0.8623. Switch it to degrees (DEG): cos 255° = −(√6 − √2)/4 ≈ −0.2588. In radians the value is never undefined: π is irrational, so a whole number of radians never lands exactly on an axis.

See Also