### Instruction: The first three terms of an arithmetic sequence are 1, 10 and 19, respectively. What is the value of the 21st term?### Response: An arithmetic sequence is one where each term is obtained by adding a constant amount to the previous term.
This constant amount is called the common difference, and I can find it by subtracting any term from the next one.
For example, 10 - 1 = 9 and 19 - 10 = 9, so the common difference is 9.
To find the value of any term in an arithmetic sequence, I can use a formula that involves the first term, the common difference, and the position of the term in the sequence.
The formula is a_n = a_1 + (n - 1)d, where a_n is the nth term, a_1 is the first term, and d is the common difference.
I want to find the value of the 21st term, so I plug in a_1 = 1, d = 9, and n = 21 into the formula.