Formula:
a_1)}{2} [/tex]
[tex]n= \frac{a_n-a_1}{d} +1[/tex]
1. There is obviously 50 terms here.
So:
[tex]S_n= \frac{n(a_n+[tex]S_{50}= \frac{50(50+1)}{2} =25*51=1275[/tex]
2. The first term would be 4, and the last would be 80. The common difference is 4.
[tex]n= \frac{80-4}{4} +1= 20[/tex]
[tex]S_{20}= \frac{20(80+4)}{2} =840[/tex]
