Tentukan banyak suku dari barisan berikut! 6+9+12+15+...=756 56+51+46+41+...=-36 10+14+18+22+...=640

56+51+46+41+...=-36
a = 56
b = 51-56 = -5
Sn = -36

Sn = n/2 (2a+(n-1)b)
-36 = n/2 (2.56+(n-1)-5)
-36 = n/2 (112-5n+5)
-36 = n/2 (117-5n) --> kdua ruas dikali 2
-72 = n (117-5n)
-72 = 117n-5n²
5n²-117n-72 = 0
(5n+3)(n-24) = 0
n = -3/5 (tdk mgkin)
n = 24
jd banyak suku ada 24