A geometric sequence is a sequence (that is an ordered list of numbers, usually following some pattern). The nth number in this sequence (a_n) can be generated by multiplying the base a by r^n, so a_2 would be a*r^2. Hope that makes it clear!

right, but have you considered that (targeted!) terrorism works?

while this is most likely true, you don’t really know what other sources of lead you come into contact with and it is probably a bad idea to add to the list deliberately