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b = 70 - 80
b= -10
sn = n/2 (2a+(n-1)b)
S60 = 60 /2 (2.80 (60-1) (-10)
= 30 (160 + 59 x (-10) )
= 30 (160 - 590)
=30 . (-430)
= - 1290