Prove that a triangle is isosceles if one of the angles is 40 degrees and one of the outer angles is 110 degrees.
September 25, 2021 | education
| 1. Let us introduce the designation of the vertices of the triangle by the symbols A, B, C, and the angles by the symbol ∠.
∠ А = 40 °. Outside apex angle B = 110 °.
2. We calculate the value of the inner angle at the vertex B, adjacent to the outer one, equal to 110 °:
∠В = 180 ° – 110 ° = 70 °.
3. We calculate the value of ∠С, based on the fact that the total value of all internal angles of the triangle is 180 °:
∠С = 180 ° – ∠А and ∠В = 180 ° – 40 ° – 70 ° = 70 °.
4. Angles B and C at the base of BC are equal. Consequently, the triangle ABC is isosceles, as required.
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