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
