Cos 225 Degrees

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

What is cos 225°?

Answer

cos 225° =
−√2/2 ≈ −0.7071

Angle in radians: 225° = 5π/4. The point of the angle is in quadrant III, where cosine is negative.

≈ — rounded to 4 decimal places.

cos 225° = cos(180° + 45°) = −cos 45° = −√2/2

P(cos 225°, sin 225°) = (−√2/2, −√2/2)

An isosceles right triangle with legs 1: the hypotenuse is √2.

cos 45° = 1/√2 = √2/2

xyOP

P(cos 225°, sin 225°) ≈ (−0.7071, −0.7071)

Highlighted: the projection on the x-axis — cosine, x ≈ −0.7071.

All four functions of 225°
FunctionExact valueDecimal
sin 225°−√2/2≈ −0.7071
cos 225°−√2/2≈ −0.7071
tan 225°11
cot 225°11

Reference angle: cos 45° = √2/2

See Also

Recalculate with different values ↑

Cosine of nearby angles

Anglecos
211°≈ −0.8572
215°≈ −0.8192
216°−(√5 + 1)/4
225°−√2/2
227°≈ −0.6820
234°−√(10 − 2√5)/4
240°−1/2

FAQ

What is cos 225°?

cos 225° = −√2/2 ≈ −0.7071

How to remember sin 30°, sin 45° and sin 60°?

Write them as √1/2, √2/2, √3/2: sin 30° = 1/2, sin 45° = √2/2, sin 60° = √3/2. Cosines are the same numbers in reverse order: cos 30° = √3/2, cos 45° = √2/2, cos 60° = 1/2; tan = sin / cos and cot = cos / sin. Here: cos 225° = −√2/2 ≈ −0.7071.

Why does a calculator show 0.3673 for cos 225?

The calculator is in radian mode (RAD): it takes cos 225 as the cosine of 225 radians, cos 225 rad ≈ 0.3673. Switch it to degrees (DEG): cos 225° = −√2/2 ≈ −0.7071. In radians the value is never undefined: π is irrational, so a whole number of radians never lands exactly on an axis.

See Also