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= 27 + 3b = 18
= 3b = 18-27
= 3b = - 9
= b = -3
Sn = n/2 (2.a +(n-1)b)
= 20/2 ( 2.27+(20-1)-3)
= 10(54+(19)(-3))
= 10 ( 54 + (-57)
= 10(-3)
= -30