Consider $a$ triangle ABC with ∠A = 45° and ∠B = 5°. What is the type of ΔABC?

Step 1 (Angle Sum Theorem): The sum of the interior angles of any triangle is always $180^{\circ }$:

Step 2 (Solve for the Third Angle): Substituting the given values $\angle A=45^{\circ }$ and $\angle B=5^{\circ }$:

Step 3 (Classify Triangle): An obtuse triangle is defined as a triangle with one interior angle strictly greater than $90^{\circ }$. Since $\angle C=130^{\circ }>90^{\circ }$, $△ABC$ is an obtuse-angled triangle.

Step 4 (Conclusion): The type of $△ABC$ is obtuse.