Question 1084810
<pre>
{{{a[n]=a[1]+(n-1)d}}}
</pre>
its 5th term is 33
<pre>
{{{a[5]=a[1]+(5-1)d}}}
{{{33=a[1]+4d}}}
</pre>
the 20th term is 123
<pre>
{{{a[20]=a[1]+(20-1)d}}}
{{{123=a[1]+19d}}}

Solve the system:

{{{system(a[1]+4d=33,a[1]+19d=123)}}}

a<sub>1</sub> will be the first term and d will be
the common difference.  You finish.

Edwin</pre>