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Verified answer
Bab Barisan dan DeretMatematika SMP Kelas IX
Sn = (n/2) x (2a + (n - 1) b)
S7 = (7/2) x (2a + (7 - 1) x b) = 18
2a + 6b = 18 x 2/7
2a + 6b = 36/7
4a + 12b = 72/7
S13 = (13/2) x (2a + (13 - 1) x b) = 20
2a + 12b = 20 x 2/13
2a + 12b = 40/13
2a + 12b = 40/13
4a + 12b = 72/7
-------------------------- -
-2a = -656/91
a = 328/91