等比数列an a1+a2+a3=18 a2+a3+a4= -9 求 limSn=?
人气:195 ℃ 时间:2020-01-27 18:30:28
解答
(a2+a3+a4)/(a1+a2+a3)=q=-(1/2)
a1+a2+a3=a1+(-1/2)a1+(1/4)a1=(3\4)a1=18
a1=24,q=-(1/2)
Sn=(a1(1-(-1/2)^n))\(1+1/2)=16-16(-1/2)^n
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