hitunglah jumlah deret aritmetika 15+12+9...+(-36)

beda = -3
a = 15

Un = a + (n-1)b
-36 = 15 + (n-1) -3
-36 = 15 + -3n + 3
-36 - 18 = -3n
-54 = -3n
n = -54/-3
n = 18

Sn = n/2 . (2a + (n – 1)b)
S18 = 18/2 . ( 2.15 + 17b)
S18 = 9 . ( 30 + 17.-3)
S18 = 9. (-21)
S18 = -189

-wijayaant-