Find the infinite sum of f(a, b, c) = ((2 / 5) ^ a) * ((11) ^ (-c)) * ((7 ^ (-a - b))
for ordered triples(a, b, c) such that a, b, c satisfy the triangle inequality. I simplified the input to a simpler formula given above but unable to approach further. Answer can be represented in the form p / q. Here a, b, c must be positive integers.
I am unable to put this condition to calculate this sum.