It is the 2 sides which are opposite the 2 equal base angles which are equal in length. Isosceles triangles are those triangles that have two sides of equal measure. Make sure that you get the equal sides and angles in the correct position. The common mistake is identifying the wrong sides as the equal (congruent sides). Seeing the triangles in different positions will help with this understanding.įor example, here is a picture where the base angles of an isosceles triangle are on the top. The common mistake is thinking that the base of the angles are always on the bottom of the isosceles triangle. The word isosceles is pronounced 'eye- sos -ell-ease' with the emphasis on the 'sos'. Notice it always remains an isosceles triangle, the sides AB and AC always remain equal in length. Try this Drag the orange dots on each vertex to reshape the triangle. So when students classify the triangles, they wind up classifying them incorrectly. A triangle which has two of its sides equal in length. However, equilateral triangles have three equal (congruent) sides and angles and can be classified as isosceles.Ī common mistake when classifying triangles is mixing up the definitions of acute angle and obtuse angle. Isosceles triangles only have two equal (congruent) sides and angles and cannot be classified as equilateral. Understanding that properties of isosceles triangles and equilateral triangles can help with questions like this. The easy mistake to make is stating that isosceles triangles can be classified as equilateral triangles. Thinking that isosceles triangles can be classified as equilateral trianglesĪ question may ask students to explain if an isosceles triangle can be equilateral.

Define isosceles triangle in math