So the chance that it doesn't come up on the d6 is 5/6. The chance that it doesn't come up the next time on a d7 is 6/7. So the combined chance that it doesn't come up after 2 iterations is 5/6 * 6/7 = 5/7. The next iteration has 7/8ths chance that '1' doesn't come up, so the combined chance for 3 rolls is 5/7 * 7/8 = 5/8. Keep repeating this and the numerator stays 5, while the denominator goes to infinity - becoming less and less likely that '1' will not ever come up, so more and more likely that '1' WILL come up.