Question

A triangle has side lengths of 10, 11, and 15 . what type of triangle is it?

Answer 1

Answer: scalene and obtuse

Justification:

You can find the angles using the law of cosine:

c^2 = a^2 + b^2 - 2abcos(γ)

=> cos(γ) = [a^2 + b^2 - c^2] / (2ab)

1) cos(γ) = [10^2 + 11^2 - 15^2] / (2*10*11) = - 0.0181818

=> γ = arccos(-0.0181818} ≈ 91°

2) cos(α) = [b^2 + c^2 - a^2 ] / 2bc = [11^2 + 15^2 - 10^2] / (2*11*15] = 0.7454545

=> α = arccos(0.7454545) ≈ 41.8°

3) cos(β) = [a^2 + c^2 - b^2] / (2ac) = [10^2 + 15^2 - 11^2] /(2*10*15) = 0.68

=> β = arccos(0.68) ≈ 47.2°

4) Verification: 91° + 41.8° + 47.2° = 180°

5) The triangles with the three different sides are called scalenes (which you can tell with only the measures of the sides).

6) The triangles with one angle greater than 90° are called obtuse.

So, the triangle is scalene and obtuse.