作业帮若两个等差数列{an} {bn} 满足a1+a2+a3+.+an/b1+b2+b3+.+bn=7n+2/n+3 求a5/b5我用两种方法算,结果不一样...帮我看看第二种哪里错了呗第一种：(a1+a2+a3+.+an)/(b1+b2+b3+.+bn)=7n+2/n+3San/Tbn=(7n+2)/(n+3)a5/b5=[1/2*(a1+a9)]/
来源：学生作业帮助网 编辑：作业帮 时间：2024/11/28 03:34:04
![若两个等差数列{an} {bn} 满足a1+a2+a3+.+an/b1+b2+b3+.+bn=7n+2/n+3 求a5/b5我用两种方法算,结果不一样...帮我看看第二种哪里错了呗第一种：(a1+a2+a3+.+an)/(b1+b2+b3+.+bn)=7n+2/n+3San/Tbn=(7n+2)/(n+3)a5/b5=[1/2*(a1+a9)]/](/uploads/image/z/5091360-24-0.jpg?t=%E8%8B%A5%E4%B8%A4%E4%B8%AA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D+%7Bbn%7D+%E6%BB%A1%E8%B6%B3a1%2Ba2%2Ba3%2B.%2Ban%2Fb1%2Bb2%2Bb3%2B.%2Bbn%3D7n%2B2%2Fn%2B3+%E6%B1%82a5%2Fb5%E6%88%91%E7%94%A8%E4%B8%A4%E7%A7%8D%E6%96%B9%E6%B3%95%E7%AE%97%2C%E7%BB%93%E6%9E%9C%E4%B8%8D%E4%B8%80%E6%A0%B7...%E5%B8%AE%E6%88%91%E7%9C%8B%E7%9C%8B%E7%AC%AC%E4%BA%8C%E7%A7%8D%E5%93%AA%E9%87%8C%E9%94%99%E4%BA%86%E5%91%97%E7%AC%AC%E4%B8%80%E7%A7%8D%EF%BC%9A%28a1%2Ba2%2Ba3%2B.%2Ban%29%2F%28b1%2Bb2%2Bb3%2B.%2Bbn%29%3D7n%2B2%2Fn%2B3San%2FTbn%3D%287n%2B2%29%2F%28n%2B3%29a5%2Fb5%3D%5B1%2F2%2A%28a1%2Ba9%29%5D%2F)
若两个等差数列{an} {bn} 满足a1+a2+a3+.+an/b1+b2+b3+.+bn=7n+2/n+3 求a5/b5我用两种方法算,结果不一样...帮我看看第二种哪里错了呗第一种：(a1+a2+a3+.+an)/(b1+b2+b3+.+bn)=7n+2/n+3San/Tbn=(7n+2)/(n+3)a5/b5=[1/2*(a1+a9)]/
若两个等差数列{an} {bn} 满足a1+a2+a3+.+an/b1+b2+b3+.+bn=7n+2/n+3 求a5/b5
我用两种方法算,结果不一样...帮我看看第二种哪里错了呗
第一种：(a1+a2+a3+.+an)/(b1+b2+b3+.+bn)=7n+2/n+3
San/Tbn=(7n+2)/(n+3)
a5/b5=[1/2*(a1+a9)]/[1/2*(b1+b9)]
=9*[1/2*(a1+a9)]/9*[1/2*(b1+b9)]
=S9/T9
=(7*9+2)/(9+3)
=65/12
第二种：看做a1+a2+...+an=7*n的平方+2n; b1+b2+...+bn=n的平方+3n
则a5/b5=4.625
若两个等差数列{an} {bn} 满足a1+a2+a3+.+an/b1+b2+b3+.+bn=7n+2/n+3 求a5/b5我用两种方法算,结果不一样...帮我看看第二种哪里错了呗第一种：(a1+a2+a3+.+an)/(b1+b2+b3+.+bn)=7n+2/n+3San/Tbn=(7n+2)/(n+3)a5/b5=[1/2*(a1+a9)]/
第一种是对的,不求a5和b5,可直接用条件求得；
第二种方法,开始也对,但可以用等差数列求和公式,进一步求出a1和d,a1=9,d=14,a5=65；
b1=4,d=2,b5=12；a5/b5=65/12