In #17, the first term is 2 and the second is 10. 10 is 5 times 2. Checking further, you will conclude that the "common ratio" is 5.
Then a_(n) = 2*5^(n-1), with n beginning at 1.
Check: what is a1? It's 2*5^(1-1) = 2, which is what we expected.
What is the 3rd term? It's 2*5(3-1) = 2*5^2 = 50, which is what we expected.
