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by baulan on April 8th, 2007

baulan

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How do you calculate the angles of an Isosceles triangle, when you already know the lengths? (i.e. the legs are 15cm and the base is 13cm). Please treat me like an idiot, I really don't understand maths.

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  • by Quirkie on April 9th, 2007

    Quirkie

    Divide the isoceles triangle in half.

    You then have a triangle with a right-angle, with base 6.5cm and longest side 15cm.
    The angle joining these two sides was in the original triangle. It is next to the "hypotenuse" - the longest side and the "adjacent" (side).
    The SOH-CAH-TOA rule says that in a triangle with a right-angle, the Cosine of an angle is the Adjacent over the Hypotenuse.
    Divide adjacent 6.5 by hypotenuse 15 to get 0.433
    This is the cosine of the angle we want.
    Use a calculator to the "inverse cosine" of 0.433
    Put the calculator in degrees mode.
    Enter 0.433
    Press "2nd" (or "Inv")
    Press "Cos"
    Answer 64.3 degrees.
    As the isoceles triangle is symmetric, the other bottom angle will be 64.3 degrees too.
    Angles in a triangle sum to 180 degrees
    Top angle is 180 - 64.3 - 64.3 = 51.4

    Comments

    • Good answer. +6

      Halskiisaklink

      by Halskiisaklink on April 10th, 2007

    • Very well explained, you put it across very easy... thank u very much

      Mags

      by Mags on December 2nd, 2009

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