Answer: Any isosceles triangle is a counter example. More specifically, a triangle with sides 7, 7 and 3
When forming your triangle, make sure you apply the triangle inequality theorem. This is the idea where adding any two sides leads to a result larger than the third side. So we have
7+7 = 14 which is larger than 3
7+3 = 10 which is larger than 7
By definition, an isosceles triangle has two congruent sides. Some books say "at least 2 congruent sides", but I'll go with the first definition. If you want all three sides to be congruent, then you'd go for the term "equilateral".
You could make use of the formula for the sum of the terms, or you could work out the sum from the given expression. It is easier just to list the terms and and them up.
