Jawabanmu

2016-07-10T12:58:09+07:00
U7=22
U11=34
b=U11-U7/11-7
b=34-22/4=12/4=3

Sn=n/2[2(U7)+(n-7)b]
Sn=18/2[2*22+(18-7)3]
Sn=9[44+(11)3]
Sn=9[44+33]
Sn=9[77]
Sn=693