classify triangle based on side lengths

Question

 7, 15, and 21. choices are right, acute, obtuse or no tirangle

Answer 1

Use Pythagorean theorem c² = a² + b² to classify the triangle.

If c² = a² + b² then the triangle is right triangle

If c² < a² + b² then the triangle is acute triangle

If c² > a² + b² then the triangle is obtuse triangle

21² = 7² + 15²

441 = 49 + 225

441>274

So triangle is obtuse triangle