Question 944285
Hello,
How can I find {{{a[1]}}} and d if
{{{ a[4]=16 }}} and {{{ s[15]=420  }}} ?

I am not sure which formula to use.
Thanks so much.
<pre><font face = "Tohoma" size = 4 color = "indigo"><b>Use the "term" formula to create an equation in the 2 variables, {{{a[1]}}} and {{{d}}}
Term formula: {{{a[n] = a[1] + (n - 1)d}}}
Use the "sum of terms" formula to create an equation in the 2 variables, {{{a[1]}}} and {{{d}}}
Sum of terms formula: {{{S[n] = (n/2)(2a[1] + (n - 1)d)}}}

From the 2 equations just formed, solve for the 2 variables, {{{a[1]}}} and {{{d}}}

You should get: 1<sup>st</sup> term, or {{{highlight_green(a[1] = 7)}}}  
You should get: common difference, or {{{highlight_green(d = 3)}}}   
Then do a check!! 
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