What is the 10th term in the sequence below? - 1, 1, 3, ... Write your answer as a number.

We notice that the difference between the 2nd term & the first is equal to the difference of the 3rd with the second:

1 - (-1) = (3-1) = 2. So d (common difference) = 2

This is an arithmetic progression with first tem a₁ = (-1) & d = 2

Calculating the nth term: Last term = a₁ + (n-1) d

10th term = (-1) + (10-1) (2) = - 1 + (9x2) = 17

-1 plus 2 equals 1. 1 plus 2 equals 3 ...