{an},{bn}是两个等差数列,其前n项和分别为Sn和Tn,且Sn/Tn=(7n+2)/(n+3),求a5/a6
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{an},{bn}是两个等差数列,其前n项和分别为Sn和Tn,且Sn/Tn=(7n+2)/(n+3),求a5/a6
∵{an},{bn}是两个等差数列
Sn/Tn=(7n+2)/(n+3)
∴[n/2(a1+an)]/[n/2(b1+bn)]=(7n+2)/(n+3)
∴(a1+an)/(b1+bn)=(7n+2)/(n+3)
∴a5/b5=(2a5)/(2b5)=(a1+a9)/(b1+b9)
=(7×9+2)/(9+3)=65/12
求a5/a6缺条件:
再问: 想法求出an啊 但是怎么求
再答: Sn/Tn=(7n+2)/(n+3)约掉的是kn Sn=kn(7n+2) a6=S6-S5=264k-185k=79k a5=S5-S4=185k-120k=65k a5/a6=65/79
Sn/Tn=(7n+2)/(n+3)
∴[n/2(a1+an)]/[n/2(b1+bn)]=(7n+2)/(n+3)
∴(a1+an)/(b1+bn)=(7n+2)/(n+3)
∴a5/b5=(2a5)/(2b5)=(a1+a9)/(b1+b9)
=(7×9+2)/(9+3)=65/12
求a5/a6缺条件:
再问: 想法求出an啊 但是怎么求
再答: Sn/Tn=(7n+2)/(n+3)约掉的是kn Sn=kn(7n+2) a6=S6-S5=264k-185k=79k a5=S5-S4=185k-120k=65k a5/a6=65/79
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