Question 1027002
By definition of an arithmetic progression,
{{{a[n]=a[1]+(n-1)*d}}}
.
.
{{{a[4]=a[1]+(4-1)*d}}}
.
.
Substituting,
{{{20=a[1]+(4-1)*4}}}
{{{20=a[1]+3*4}}}
{{{20=a[1]+12}}}
{{{highlight(a[1]=8)}}}
So then,
{{{a[n]=8+(n-1)*4}}}
{{{a[n]=8+4n-4}}}
{{{highlight(a[n]=4+4n)}}}
{{{a[1]=8}}}
{{{a[2]=12}}}
{{{a[3]=16}}}
{{{a[4]=20}}}