diket.deret aritmetika 60+50+40... jumlah sampai 15 suku pertamanya adalah??

60+50+40 ....

a = 60

b= 50 - 60 = -10

Sn= n/2 (2a+(n-1)b)

s15 = 15/2 ( 2.60 + (15-1).-10)

s15 = 7,5 ( 120 + (14.-10))

s15 = 7,5 ( 120 - 140)

s15 = 7,5 . - 20

s15 = -150