等比數列,a1+a2+a3=6 a2+a3+a4=-3,怎麼求公比和首項
- 2022-12-31
an=a1+(n-1)d
a1+a2+a3
=3a1+3d=6
a2+a3+a4
=3a1+6d=-3
解得a1=5
d=-3
因為a1+a2+a3=6 q(a1+a2+a3)=a2+a3+a4=-3 所以q=-1/2 代入a1(1+q+q>2)=6可求a1=8
a2+a3+a4=q(a1+a2+a3)=-3
6q=-3
q=-1/2
a1+a2+a3=a1(1+q+q^2)=6
a1=8
a1+a2+a3=6 a1+a1q+a1q^2=6
a1(1+q+q^2)=6
a2+a3+a4=-3 a1q+a1q^2+a1q^3=-3
a1q(1+q+q^2)=-3
a1(1+q+q^2)/a1q(1+q+q^2)=6/-3
q=-2 a1=2