aritmatika nih tolong donk kak soal no 3

U10 = 38
a + 9b = 38
2 + 9b = 38
9b = 38 - 2
9b = 36
b = 36/9
b = 4

Sn = n/2 (a+ Un)
Sn = n/2 (a + a+(n -1)b)
S20= 20/2 (2 + 2+(20 -1)4)
S20 = 10 (4 + 76)
S20 = 10 (80)
S20 = 800

jadi jumlah suku 20 pertama adalah 800