Tentukan nilai m jika: 5+8+11+...+m=220

jawab
deret aritmetika 
a= 5
b = 3

un = m --> m = 3n + 2

sn  = n/2 (a + un)
220 = n/2 ( 5+ 3n + 2)
440 = n(7 + 3n)
3n² + 7n - 440 = 0
(3n + 40)(n - 11) = 0
n = -40/24 (tolak  krn < 0)
n = 11
m = n= 11