hitunglah jumlah deret 15+10+5+...+200

a=15 dan b=-5
Un=a+(n-1)b
= 15+(n-1)-5
200=15-5n+5
200=5n+10
5n=200+10
n=210/5=42

Sn=n/2(2a+(n-1)b)
Sn=42/2(2(15)+(42-1)-5
=42/2(30)+(41)-5
=42/2(30-205)
=42/2(-175)
=21(-175)= -3675