x54 = (x55 + x 53) / 2 = (550 + 530) / 2 = 540
d = x54 - x 53 = 540 - 530 = 10
x1 = x53 - d * (53 - 1) = 530 - 10 * 52 = 530 - 520 = 10
x54 = (x55 + x 53) / 2 = (550 + 530) / 2 = 540
d = x54 - x 53 = 540 - 530 = 10
x1 = x53 - d * (53 - 1) = 530 - 10 * 52 = 530 - 520 = 10