tolong dibantu!tentang deret aritmetika

kalo gak salah itu jawabannya

Eliminasi a
U11 = a + 10b = 33
U5 = a + 4b = 15
-------------------------- -
6b = 18
b = 3

Subtitusi nilai b ke salah satu persamaan
a + 4b = 15
a + 12 = 15
a = 15 - 12
a = 3

Sn = n/2 [2a + (n-1)b]
S20 = 20/2 [2.3 + (20-1).3]
S20 = 10 (6 + 57)
S20 = 630

Semoga membantu ^-^