Question 1022735
No with your formula the 8th term is 25 but the 20th term is -17.
{{{a[8]=a[1]+(8-1)*d=a[1]+7d=25}}}
{{{a[20]=a[1]+(20-1)*d=a[1]+19d=-11}}}
So then,
{{{a[20]-a[8]=-11-25}}}
{{{a[1]+19d-a[1]-7d=-36}}}
{{{12d=-36}}}
{{{d=-3}}}
and
{{{a[1]+7(-3)=25}}}
{{{a[1]-21=25}}}
{{{a[1]=46}}}
So then,
{{{a[n]=46-3(n-1)}}}
{{{a[n]=46-3n+3}}}
{{{highlight(a[n]=49-3n)}}}
You can verify that the 8th and 20th term match what you need.