Atriangle has side lengths of 20 cm, 99 cm, and 108 cm. classify it as acute, obtuse, or right.

a. acute
b. obtuse
c. right

B. Obtuse

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

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.