设a2=a,a3=aq,a4=aq^2,a5=aq^3,a6=aq^4a2*a4+2a3*a5+a4*a6=a*aq^2+2aq*aq^3+aq^2*aq^4=a^2(q^2+2q^4+q^6)=a^2*q^2(1+2q^2+q^4)=(aq(1+q^2))^2=(aq+aq^3))^2=(a3+a5)^2=25又an