Angles in an Isosceles Triangle

Primary 5 Properties of Triangles · Lesson video · 2 min 17 s

Primary 5 angles in a triangle questions: the three angles make 180°, so find the missing one in right-angled, isosceles and equilateral triangles.

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Questions in this lesson

  1. 0:05 Today's question: ABC is a triangle and AB and AC are the same length. Angle BAC is 76°. Find angle ABC.
  2. 1:06 Example 1: DEF is a triangle and DE and DF are the same length. Angle EDF is 96°. Find angle x.
Transcript

A B C is a triangle and A B and A C are the same length. Angle B A C is 76 degrees. Find angle A B C. The two marks say A B and A C are the same length. So the two angles opposite them are the same size. And the three angles in a triangle add up to one hundred and eighty degrees. The top angle is 76 degrees. The other two share what is left after it. So angle A B C is one hundred and eighty minus 76, all divided by 2, which is 52 degrees. The bracket says which property gave it. Now three equal sides. All three angles are equal, and they share the same one hundred and eighty degrees. So each one is 60 degrees. That is the equilateral triangle.

Your turn. D E F is a triangle and D E and D F are the same length. Angle E D F is 96 degrees. Find angle x. Try it first. Pause if you like. D E and D F are the same length, so the two angles opposite them are the same size. So angle x is one hundred and eighty minus 96, all divided by 2, which is 42 degrees. And the bracket says which property gave it. Two equal sides, and two equal angles. The three angles in a triangle are one hundred and eighty degrees together. 96 degrees of that is the top angle. Equal sides face equal angles, and the three angles of a triangle add up to one hundred and eighty degrees.