tolong ya, makasih jangan lupa pakai caranya, yg nomor 38&39

39.
a:10000
b:300
n:20
Sn :1/2n(2a+(n-1)b)
:1/2.20(2.10000+(20-1)3000
:10(20000+19.3000)
:10(20000+57000)
:10(77000)
:770000