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U7 = 19 ---> a+6b = 19 ... (2)
eliminasi,
a + 4b = 14
a + 6b = 19
----------------- (-)
-2b = -5
b = 5/2
substitusi,
a + 4b = 14
a + 4(5/2) = 14
a + 10 = 14
a = 2
jadi, U1 = a = 2 dan beda (b) = 5/2
U10 = a + 9b = 2 + 9(5/2) = 2 + 45/2 = 49/2
U20 = a + 19b = 2 + 19(5/2) = 2 + 95/2 = 99/2
jumlah 8 suku :
S8 = 8/2 (2a + 7b) = 4(2(2) + 7(5/2)) = 4 (4 + 35/2) = 86