Which set of numbers could represent the lengths of sides of an obtuse triangle?

Question

Which set of numbers could represent the lengths of sides of an obtuse triangle?

a	4,5,6
b	5,10,15
c	5,12,15
d	30,40,50

Answer 1

In an obtuse triangle the sum of the squares of the lengths of the 2 shorter sides is length than the square of the length of the longest side:

4^2 + 5^2 = 41; 6^2 = 36 -> not obtuse (acute)

5^2 + 10^2 = 125; 15^2 = 225 -> obtuse

5^2 + 12^2 = 169; 15^2 = 225 --> obtuse

30^2 + 40^2 = 2500; 50^2 = 2500 --> not obtuse (right triangle)