b1+b1q=20; b1=20/(1+q)
b1q+b1q^2=b1(q+q^2)=20*(q+q^2)/(1+q)=20/3
60(q+q^2)=20+20q
60q^2+40q-20=0
3q^2+2q-1=0
D=16
q1=(-2+4)/6=1/3
q2=(-2-4)/6=-1-не подходит
b1=20/(1+1/3)=15
S=b1/(1=q)=15/(2/3)=22.5
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b1+b1q=20; b1=20/(1+q)
b1q+b1q^2=b1(q+q^2)=20*(q+q^2)/(1+q)=20/3
60(q+q^2)=20+20q
60q^2+40q-20=0
3q^2+2q-1=0
D=16
q1=(-2+4)/6=1/3
q2=(-2-4)/6=-1-не подходит
b1=20/(1+1/3)=15
S=b1/(1=q)=15/(2/3)=22.5