Jawabanmu

2014-04-21T13:21:31+07:00
Sn=n/2(a+Un)……3(a+12)=12 Un=a+(n-1)b………a+5b=12 3a=-24 |×1| 3a=-24 a+5b=12|×3|3a+15b=36 (dieliminasi) 15b=-60…………………… b=-4......................... a+5b=-12............ a+5(-4)=-12......... a+-20=-12.......... a=8............... U15=8-56=-47