Question 1185122


{{{-5}}}, {{{-11}}}, {{{-17}}}

find differences

{{{-5}}}.......... {{{-11}}}.............{{{-17}}}
.......{{{-6}}}.............{{{-6}}} 

The formula for {{{n}}}th term of an arithmetic progression is{{{ a[n]=a[1]+dn-d}}}. In this example we have {{{d=-6}}} and {{{ a[1]=-5}}}, so

{{{a[n]=-5-6n-(-6)}}}
{{{a[n]=-5-6n+6}}}
{{{a[n]=-6n+1}}}


then the {{{21}}}st term is:
 ->{{{ n=21}}}
{{{a[21]=-6*21+1}}}
{{{a[21]=-125}}}