Final answer:
To prove that triangle ABC is obtuse, we sum the measures of angles ABD and DBC which gives us angle ABC measuring 100°, indicating that the triangle is obtuse.
Step-by-step explanation:
Given the measures of angles in the triangle, we will prove that triangle ABC is an obtuse triangle step by step. The given information states that m∠ABD = 60° and m∠DBC = 40°.
- m∠ABD = 60°, m∠DBC = 40°: Given by the problem.
- m∠ABD + m∠DBC = m∠ABC: By the angle addition postulate, angles ABD and DBC are adjacent, thus their sum is equal to angle ABC.
- m∠ABC = 100°: By substituting the given values for m∠ABD and m∠DBC and simplifying (60° + 40°).
- ∠ABC is an obtuse angle because it measures more than 90° but less than 180°.
- Since ∠ABC is obtuse, by the definition of an obtuse triangle, triangle ABC is classified as an obtuse triangle because it contains one obtuse angle.