Question

If each angle of a triangle is less than the sum of the other two angles of it; prove that the triangle is acute-angled.

Answer 1

Consider ΔABC.
Now, ∠A < ∠B + ∠C
⇒ ∠A + ∠A < ∠A + ∠B + ∠C
⇒ 2∠A < 180°
⇒ ∠A < `(180°)/(2)`
⇒ ∠A < 90°
Similarly, we have
∠B < 90° and ∠C < 90°.
Hence, the triangle is acute-angled.