tentukan suku ke 12 dan jumlah 12 suku pertamanya...

U10 = 19
U7 = 13

b = (U10 - U7)/(10 - 7)
b = (19 - 13)/3
b = 2

U10 = 19
a + 9b = 19
a = 1

U12 = a + 11b = 1 + 22 = 23

S12 = n/2 (a + U12)
S12 = 12/2 (1 + 23)
S12 = 6 . 24
S12 = 144