Question 1169306


the arithmetic sequence formula :

{{{a[n]=a[1]+(n-1)d}}} where {{{a[n]}}} is n-th term, {{{a[1]}}} is 1-th term , {{{n}}} is total number of terms,  and {{{d}}} is difference between two consecutive terms

in yuor case

{{{a[1]=21}}}

{{{d=-3}}}

you need the twentieth term, so {{{n=20}}}

so, you have:

{{{a[20]=21+(20-1)(-3)}}}

{{{a[20]=21-(19)(3)}}}

{{{a[20]=21-57}}}

{{{a[20]=-36}}}

 the twentieth term of the arithmetic sequence {{{21}}}, {{{18}}}, {{{15}}}, {{{12}}} is {{{-36}}}