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    Cos 78 Degrees

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

    What is cos 78°?

    Answer

    cos 78° ≈
    0.2079

    Angle in radians: 78° = 13π/30. The point of the angle is in quadrant I, where cosine is positive.

    ≈ — rounded to 4 decimal places. An exact form of cos 78° in square roots exists, but it is unwieldy — nested radicals.

    P(cos 78°, sin 78°) ≈ (0.2079, 0.9781)

    xyOP

    P(cos 78°, sin 78°) ≈ (0.2079, 0.9781)

    Highlighted: the projection on the x-axis — cosine, x ≈ 0.2079.

    All four functions of 78°
    FunctionExact valueDecimal
    sin 78°—≈ 0.9781
    cos 78°—≈ 0.2079
    tan 78°—≈ 4.7046
    cot 78°—≈ 0.2126

    See Also

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    https://calculat.io/en/number/trigonometry/cos--78Copy
    <a href="https://calculat.io/en/number/trigonometry/cos--78">Cos 78 Degrees: Cosine of 78° on the Unit Circle</a>Copy
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    Cos 78 Degrees: Cosine of 78° on the Unit Circle. cos 78° ≈ 0.2079. The cosine is the x-coordinate of the point at 78° on the unit circle.
    https://calculat.io/en/number/trigonometry/cos--78/generated.jpgCopy

    Cosine of nearby angles

    Anglecos
    75°(√6 − √2)/4
    76°≈ 0.2419
    77°≈ 0.2250
    78°≈ 0.2079
    79°≈ 0.1908
    80°≈ 0.1736
    81°≈ 0.1564
    Recalculate with different values ↑

    FAQ

    What is cos 78°?

    cos 78° ≈ 0.2079

    Is there an exact value of cos 78°?

    Yes: 78° 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 78° ≈ 0.2079.

    Why does a calculator show −0.8578 for cos 78?

    The calculator is in radian mode (RAD): it takes cos 78 as the cosine of 78 radians, cos 78 rad ≈ −0.8578. Switch it to degrees (DEG): cos 78° ≈ 0.2079. In radians the value is never undefined: π is irrational, so a whole number of radians never lands exactly on an axis.

    See Also