Question

Find the 21st term in the following arithmetic sequence
-1, -3, -5, -7,

Answer 1

First of all we can find what it’s going down in as it’s taking away 2 each time so we know it’s -2n then we can do 2 add -1 which is 1 and know that the sequence is -2n + 1 then we put n = 21 and do -2 x 21 + 1 which gives you the answer of:
-41

Answer 2

Answer: -41

Work Shown:

`a_1 = -1 = \text{ first term}\\\\d = -2 = \text{ common difference}\\\\a_n = \text{ nth term of arithmetic sequence}\\\\a_n = a_1 + d(n-1)\\\\a_(21) = -1 + (-2)(21-1)\\\\a_(21) = \boldsymbol{-41}\\\\`