tolong ya nomor 17 bagaimana

a=104
Un=996
b=4

Un=a+(n-1)b
996=104+(n-1)4
996-104=(n-1)4
892=(n-1)4
223=n-1
n=224

Sn=n/2(a+Un)
S224=224/2(104+996)
=112(1.100)
=123.200

semoga membantu.....