设它的首项为a1,公比为q
前3项和是-3/5
则a1(1-q^3)/(1-q)=-3/5 (1)
前6项的和是21/5
则a1(1-q^6)/(1-q)=21/5 (2)
(2)/(1) 1+q^3=-7 q^3=-8 q=-2
代入(1) a1=-1/5
它的前10项的和S10=a1(1-q^10)/(1-q)
=(-1/5)*[1-(-2)^10]/(1+2)
=(1/15)(2^10-1)
=(2^10-1)/15
=1023/15