you start with an equilateral triangle and then trisect each of the sides with a new equilateral triangle on the middle third of each trisection, repeat the process indefinitely

how would you prove that the perimeter is infinite?

1 answer

You start out with three sides of length a, and the figure perimeter is 3a.

After the first step you described, you have a figure with 12 sides of length a/3. The perimeter is now 4a.

After a similar second step, you will have 48 sides of length a/9. The perimeter is now 5.33 a

After N such steps, you have 3*4^N sides of length a/3^N, with a perimeter of = 3a*(4/3)^N

That number clearly becomes infinite as N becomes infinite