Question

Tentukan banyak suku dari barisan berikut !
a) 56+51+46+41+........ = -36
b) 10+14+18+11+........= 640

Answer 1

A) a = 56, b = -5, n = ...???
   n/2 (2a + (n-1)b) = -36
   n/2 (2x56 + (n-1)-5) = -36
   n (112 -5n + 5) = -72
   117n - 5 = -72
   5 - 117n - 72 = 0,    (5n + 3)(n - 24) = 0
   n = 24 !!!!!
 
b)  a = 10, b = 4, n = ??
n/2(2a +(n-1)b) = 640
n(20 + (n-1)4) = 1280
n(20 + 4n - 4) = 1280
16n + 4 = 1280
+ 4n - 1280 = 0, (n +20)(n - 16) = 0
n = 16!!