Suku ke -40 barisan bilangan 12,20,30,42..adalah...

jawab

12 ...20...30....42
....+8...+10..+12
.......+2.....+2

a= 12
b = 8
c = 2

un =  a + (n-1) b + 1/2 c (n-1)(n-2)
u40 = 12 +39(8) + 1/2 (2)(39)(38)
u40 = 12 +312 + 8.892
u10 = 9.216