Find the largest prime number (in decimal form) that divides the sum, $$ 1_2 + 10_2 + 100_2 + \cdots + 100000000_2. $$
This problem involves binary numbers, which are base 2 instead of base 10. I recall that binary numbers are written using only 0s and 1s, and each digit represents a power of 2. For example, 10_2 means 2^1, 100_2 means 2^2, and 100000000_2 means 2^8. So, to find the sum of these binary numbers, I need to add the corresponding powers of 2. That is, $$ 1_2 + 10_2 + 100_2 + \cdots + 100000000_2 = 2^0 + 2^1 + 2^2 + \cdots + 2^8. $$ This is a geometric series, where the first term is 1, the common ratio is 2, and the number of terms is 9. I recall that the sum of a geometric series is given by the formula, $$ S_n = a \frac{r^n - 1}{r - 1}, $$ where a is the first term, r is the common ratio, and n is the number of terms. Applying this formula, I get, $$ S_9 = 1 \frac{2^9 - 1}{2 - 1} = \frac{512 - 1}{1} = 511. $$ So, the sum of the binary numbers is 511 in decimal form. Now, I need to find the largest prime number that divides 511. I know that 511 is not prime, because it is odd and divisible by 7. To see this, I can divide 511 by 7 using long division or a calculator. I get, $$ 511 = 7 \times 73. $$ 7 is prime, but 73 is also prime. Therefore, 73 is the largest prime factor of 511, and the answer. 