Question 825175
You are missing something. Your post does not explain what is the meaning of the number 125 you mention.
The "sum of the first term term" is like the sound of one hand clapping.
You need two hands to clap and you need to have at least two terms to add together to have a sum.
 
{{{a[1]}}}= the first term
{{{d}}}= common difference
{{{a[k]}}}= term number {{{k}}}
{{{S[n]}}}= sum of the first {{{n}}} terms
You can calculate any term number {{{k}}} from {{{k}}}, the first term, and the common difference:
{{{a[k]=a[1]+(k-1)d}}} .
You can calculate the sum of the first {{{n}} term from {{{n}}}, the first term, and the common difference:
{{{S[n]=n(2a+(n-1)d)/2}}} .
Having {{{n}}}, the sum of the first {{{n}}} terms, {{{k}}}, and the value of term number {{{k}}}, you can set up the two equations above and solve for {{{a[1]}}} and {{{d}}} .