The first three terms of an arithmetic sequence are 1, 10 and 19, respectively. What is the value of the 21st term?

1,10,19,... +9 each time

\(T_n=dn+c\) where d is a common difference (9) and c is the first term - d

\(T_n=9n-8\)

Thus the 21th term is 

\(T_{21}=9(21)-8=181\)