Given that Pi can be estimated using the function 4 * (1 – 1/3 + 1/5 – 1/7 + …) with more terms giving greater accuracy, write a function that calculates Pi to an accuracy of 5 decimal places.

And there you have encountered the biggest problem with that question. Unless you prove how and why that approximation works, the only way to know for certain that you’ve estimated to 5 digits is to compare it against a known value of Pi. That sort of defeats the point.

Once the terms get smaller than 10^-5 then the approximation must be accurate to 5 decimal places. No proof required.