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b = 15-12 = 3
Sn = n/2 (2a +(n-1)b)
S8 = 8/2 [(2 x 12) + (8-1)3]
S8 = 4 (24 + (7 x 3)
S8 = 4 (24 + 21)
S8 = 4 (45)
S8 = 180
b=3
un = a + (n-1)b
u8 = 12 + 7x3 = 12 + 21 = 33
sn = n/2(a + un)
s8=4(12+u8)
s8=4(12+33)
s8=4(45)
s8 = 180