U2=7,U5=19.jmlh deret suku ke-1 sampe ke-20

U2 = U1 + b = 7
U5 = U1 + 4b = 19
----------------------------[-]
-------------> -3b = -12
-------------> b = 4

U2 = U1 + b
7 = U1 + 4
U1 = 3

S20 = 1/2 . 20 ( 2U1 + (20-1)4))
S20 = 10 ( 6 + 19.4)
S20 = 10 ( 6 + 76)
S20 = 10 . 82
S20 = 820

semoga membantu ^_^