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Using the Law of Cosines, how can I show that the measure of each angle of an equilateral triangle is 60 degrees?

  • Trigonometry -

    since the sides are all equal,

    s^2 = s^2+s^2 - 2s^2 cosθ
    cosθ = 1/2

    since you can pick any side, the value for θ is good for all three angles.

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