a1(1+q+q²)=3a1
q²+q-2=0
(q-1)(q+2)=0
q=1或q=-2
等比数列的公比是1或-2为什么要a1(1+q+q²)=3a1前3项的和s3=a1+a2+a3a2=a1qa3=a1q²