Question 193885
ar=4    a/(1-r)=18
a=4/r   4/r=(1-r)18

4/r=(1-r)18

2/9=(1-r)r

r^2-r+2/9=0
D = 1-8/9=1/9

r1=2/3  r2=1/3
a1=6    a2=18