Question 292655
15 + 1 + (-13) + (-27)
<pre><font size = 4 color = "indigo"><b>
{{{a[2]-a[1]=(1)-(15)=-14}}}
{{{a[3]-a[2]=(-13)-(1)=-14}}}
{{{a[4]-a[3]=(-27)-(-13)=-27+13=-14}}}

So this is a 4-term arithmetic series with d=-14.

{{{a[n]=a[1]+(n-1)d}}}
{{{a[n]=15+(n-1)(-14)}}}
{{{a[n]=15-14(n-1)}}}
{{{a[n]=15-14n+14}}}
{{{a[n]=29-14n}}}

{{{sum(a[n],n=1,4)=sum((29-14n),n=1,4)}}}

Edwin</pre>