1 + 2 + 4 + 8 + ...

1+2+4+8+.... is a pattern called a series which is made up of adding numbers together. In this case, each new number is double the previous number. So the pattern looks like this:
1, 2, 4, 8, 16, 32, 64...
To keep adding to the pattern, take the last number and double it (which is 64 in this example) to get the next number. So, the next number in this pattern would be 128.