10. Terms are 2, 2*6, 2*6*6, 2*6^3, 2*6^4, ..., 2*6^k where k=0 to n: 2∑6^k for k=0 to n.
11. 1+27+27-4+27-8+...+27-32+...; this is 1+∑27-4k where k=0 to n, where n=8 if -5 is the last term.
12. Every other term is negative, so we can use (-1)^k to alternate between + and -; 2∑(-1)^k*5^k where k=0 to n.