The rule of an arithmetic sequence:
\(a_n=a_1+(n-1)d\)
\(a_1\) - first term
\(d\) - a difference \(d=a_n-a_{n-1}\)
We have:
\(a_1=20;\ a_2=17;\ a_3=14;\ a_4=11;\ a_5=8;\ ...\)
\(a_1=20;\ d=a_2-a_1\to d=17-20=-3\)
substitute:
\(a_n=20+(n-1)\cdot(-3)=20-3n+3=-3n+23\)
Answer: f(n) = -3n + 23
