q=(2k+5)/5k=(2k-1)/(2k+5)
(2k+5)^2=5k(2k-1)
4k^2+20k+25=10k^2-5k
6k^2-25k-25=0
D=625+699=35^2
k=(25+35)/12=5
b1=5k=25
b2=2*5+5=15
q=b2/b1=15/25=0.6
S=b1/(1-q)=25/(1-0.6)=100
q=(2k+5)/5k=(2k-1)/(2k+5)
(2k+5)^2=5k(2k-1)
4k^2+20k+25=10k^2-5k
6k^2-25k-25=0
D=625+699=35^2
k=(25+35)/12=5
b1=5k=25
b2=2*5+5=15
q=b2/b1=15/25=0.6
S=b1/(1-q)=25/(1-0.6)=100