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.
by 
baulan
ANSWERS: 4
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The angles of any triangle will add up to 180 degrees. Try this website for more info. http://www.mathsrevision.net/gcse/pages.php?page=17
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It is not easy for this particular triangle, you need to use trigonometry. If you have a practical project, like you are building something, it is easier to accurately draw the triangle and to measure the angles with a protractor. Unless you made a mistake and the base is also 15 cm, in which case all the angles are equal and correspondingly to 60 degrees.
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The angles of any triangle ALWAYS total 180 degrees. (Ignore the lengths of the sides!) In an isoceles triangle, 2 of the angles will be the same. (The angles created between the base and the other sides.) Use a protractor to measure them. The remaining angle will be : 180 - (total of both angles that are the same). Hope you understand this.
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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
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