Question 368522
{{{a[1]=12}}}
{{{d=a[2]-a[1]=15-12=3}}}
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{{{a[n]=a[1]+(n-1)d}}}
{{{a[n]=12+(n-1)3}}}
{{{a[n]=12+3n-3}}}
{{{highlight(a[n]=9+3n)}}}