Arithmetic sequence formula
[tex]a_n=a_1+(n-1)d[/tex]where
• aₙ: the nᵗʰ term in the sequence
,• a₁: the first term in the sequence
,• d: the common difference between terms
In this case, the first term is a₁ = -1.
The common difference is obtained by subtracting two consecutive terms, as follows:
[tex]d=-3-(-1)=-2[/tex]To find the 21st term, we need to substitute n = 21. Using this value of n and the previously found values of the other constants, we get:
[tex]\begin{gathered} a_{21}=-1+(21-1)(-2) \\ a_{21}=-1+(20)(-2) \\ a_{21}=-1-40 \\ a_{21}=-41 \end{gathered}[/tex]