Question

A triangle has side lengths of 20 cm, 99 cm, and 108 cm. Classify it as acute, obtuse, or right.

A. acute
B. obtuse
C. right

Answer 1

B. Obtuse

If draw it out in your head, it's obviously obtuse.

Answer 2

Let "a", "b" and "с"  be sides of the triangle ("с" is the longest side).
The triangle will be:
right if       a² + b² = c²
аcute if     a² + b² > c²    
obtuse if   a² + b² < c²    

We have a=20, b=99 and c=108
20² + 99² = 400 + 9801 = 10201
and
108² = 11664

10201 < 11664   ⇒  obtuse triangle.