what is the first 20 digits of pi?
3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679 ... PI/4 = 1/1 - 1/3 + 1/5 - 1/7 + ... PI/2 = 2/1 * 2/3 * 4/3 * 4/5 * 6/5 * 6/7 * ...