S10=100,s30=900,s40=?
sn=n/2(a1+an)
s10=5(a1+a10), 100=5(a1+a10), a1+a10=20
s30=15(a1+a30), 900=15(a1+a30), a1+a30=60
a1+a10=20,a1+a1+9d=20,2a1+9d=20
a1+a30=60, a1+a1+29d=60,2a1+29d=60
---
2a1+9d=20
2a1+29d=60
-----
20-9d=60-29d, 20d=40,d=2
2a1=20-9d,2a1=20-18,2a1=2,a1=1
a40=a1+39d,a40=1+39.2=1+78,a40=79
s40=40/2(1+79)=20.80=1600
s40=1600