tolong dong soal no 7 dan8

7. a=20 dan b = 5
U15 = a + (n-1).b = 20 +(15-1).5=20+14.5=20+70=90
8. a=5 dan U5=80
U5=a.r4=80 maka r4=80/5=16 maka r = 2
s5 = a(r^n-1)/r-1
= 5(2^5-1)/2-1=5(32-1)/1=5(31)=155