已知等比数列{a}的前5项和为10,前10项和为50,求这个数列的前15项和

设an=a1*q^(n-1)
sn=a1*(q^n-1)/(q-1)
s5=a1*(q^5-1)/(q-1)=10
s10=a1*(q^10-1)/(q-1)=50
两式相除：
q^5+1=5
q^5=4
a1*(q^5-1)/(q-1)=10
a1/(q-1)=10/3
s15=a1*(q^15-1)/(q-1)
=a1*(4^3-1)/(q-1)
=63a1/(q-1)
=63*(10/3)
=210