已知等比数列的前三项的和是-3/5,前六项的和是21/5,求前十项的和

如果q=1
S3=3A1=-3/5
A1=-1/5
S6=6A1=-6/5,不符合题意
∴q≠1
S3=A1×(1-q^3)/(1-q)=-3/5
S6=A1×(1-q^6)/(1-q)=21/5
两式相除
(1-q^6)/(1-q^3)=-7
(1-q^6)/(1-q^3)=(1-q^3)(1+q^3)/(1-q^3)=1+q^3=-7
q^3=-8
q=-2
A1×(1-q^3)/(1-q)=A1×(1+8)/(1+2)=-3/5
A1=-1/5
S10=A1×(1-q^10)/(1-q)=-1/5×(1-1024)/(1+2)=341/5