tolong bantu ya sama rumusnya

U4 =a+3b = 25
U8 =a+7b = 41
------------------- -
-4b = -16
b = 4

subtitusi =
a+3(4) = 25
a+12 = 25
a = 25-12
a = 13

jumlah 20 suku pertama
S20 = 20/2(2.13 +(20-1)4)
S20 = 10(26+76)
S20 = 10(102)
S20 = 1.020

maaf kalau salah.........