tolong dibantu yaaa (besok dikumpulin)

1. U3 = 4 ; U7 = 32 ; S20 = ?
Un = a + (n-1)b
U3 = a +(3-1)b
4 = a +2b ... (1)
U7 = a + (7-1)b
32 = a + 6b ... (2)
persamaan 1 dan 2 di eliminasi
a +6b = 32
a+2b = 4

4b = 28
b = 7
disubtitusi ke persamaan a +2b = 4
a + 14 = 4
a = -10

Sn = n/2 (2a + (n-1)b)
S20 = 10 (-20 + 19.7)
= 10 (-20 + 133)
= 10 (113)
1130

maaf kalau salah