tolong dibantu.. kawan sama caranya ya

19) U2 = 11
U8=29

Un=a+(n-1)b
a+7b=29
a+b =11
--------------- -
6b=18
b=18/6 = 3

a+b=11
a+3=11
a=11-3=8

Un=170
a+(n-1)b=170
8+(n-1)3=170
8+3n-3=170
5+3n=170
3n=170-5
n=165/3 = 55

Maka banyaknya suku ada 55

20) U3=16
U7=256

Un=ar^n-1
ar^6=256
ar^2=16
---------------- :
r^4=16
r=2

ar^2=16
a.2^2=16
a.4=16
a=16/4=4

U9=ar^8
=4.2^8
=4.256
=1024