等比数列{an} 设首项a1 公比=q
a1+a2+……a5=a1(1-q^5)/(1-q)=3
首项a1^2 公比=q^2
a1²+a2²……a5²=a1^2(1-q^10)/(1-q^2)=12 相除
a1(1+q^5)/(1+q)=4
a1-a2+a3-a4+a5 首项a1 公比=-q
=a1(1-(-q)^5)/(1+q)
=a1(1+q^5)/(1+q)=4