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-8 = a + 3b + a + 9b
-8 = 2a + 12b
-8 = 2.8 + 12b
-8 = 16 + 12b
12b = -24
b = -2
S₇ = 7/2 ( 2.8 + ( 7 - 1 ) -2 )
S₇ = 3,5 ( 16 - 12 )
S₇ = 3,5 × 4
S₇ = 14