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= 15/2 ( 2.1 +(15-1)2)
= 15/2 (30)
= 225
sn =n/2 (2a + (n-1) b
s15 = 15/2 (2.1+(15-1) 2
s15= 15/2 (2+28)
s15=15(2+14)
s15=15(16)
s15=240