Diketahui barisan bilangan 12,20,30,42,56 suku ke 22 adl.....??

a=12, b=8, c=2

Un= a+(n-1)b+(n-1)×(n-2) c ÷2
=12+(n-1) 8 + (n-1) × (n-2) 2 ÷ 2
=12+8n-8+n^2-3n+2×2÷2
=n^2+5n+6

U22= n^2+5n+6
=(22.22)+5.22+6
=484+110+6
=600