tentukan jumlah 20 suku pertama dan deret aritmetika a.3+9+15 + b.-20-18-8-2

a.) a = 3
b = 6
Sn = n / 2 ( 2a + ( n - 1 ) b )
S20 = 20/2 ( 2(3) + (20-1)6)
= 10 ( 6 + 114 )
S20 = 1200

b.) a = -20
b = 2
S20 = 20/2 (2(-20) + ( 20 - 1 ) 2)
= 10 ( 40 + 38 )
S20 = 780