Question

A triangle has side lengths of 28 in. 4 in, and 31 in. Classify it as acute, obtuse, or right.

Answer 1

When given 3 triangle sides, to determine if the triangle is acute, right or obtuse:

1) Square all 3 sides.

2) Sum the squares of the 2 shortest sides.

3) Compare this sum to the square of the 3rd side.

28^2 = 784

4^2 = 16

31^2 = 961

784 + 16 = 800

800 < 961

if sum > 3rd side²   Acute Triangle

if sum = 3rd side²   Right Triangle

if sum < 3rd side²   Obtuse Triangle

Therefore, it is an obtuse triangle.
Source:http://www.1728.org/triantest.htm