Sn=a1(q^n-1)/(q-1)
S10=a1(q^10-1)/(q-1)=3,a1/(q-1)=3/(q^10-1)
S30=a1(q^30-1)/(q-1)=63
相除：
S30/S10=[a1(q^30-1)/(q-1)]/[a1(q^10-1)/(q-1)]=63/3
(q^30-1)/(q^10-1)=21
q^20+q^10=20
q^10=4
s40-s20=a1(q^40-1)/(q-1)-a1(q^20-1)/(q-1)
=a1(q^40-q^20)/(q-1)
=(q^40-q^20)[3/(q^10-1)]
=3(q^40-q^20)/(q^10-1)
=3(4^4-4^2)/(4-1)
=(4^2-1)4^2
=240