Question 1073594
{{{a[n]=a[1]+(n-1)d}}}
So,
{{{a[5]=a[1]+(5-1)d=a[1]+4d=23}}}
{{{a[10]=a[1]+(10-1)d=a[1]+9d=48}}}
Subtracting the two equations,
{{{a[1]+9d-a[1]-5d=48-23}}}
{{{5d=25}}}
Solve for d, then use either equation to solve for {{{a[1]}}}.
Then find,
{{{a[18]=a[1]+17d}}}