请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的前数列之和,这一数列的规律是,它们前后之差为等请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的数列前n项和之和。这一数列的规律是,它们前后
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![请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的前数列之和,这一数列的规律是,它们前后之差为等请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的数列前n项和之和。这一数列的规律是,它们前后](/uploads/image/z/61734-30-4.jpg?t=%E8%AF%B7%E9%97%AE%E6%B1%82%E6%95%B0%E5%88%9718%2C34%2C55%2C81%2C%E2%80%A6%E2%80%A6%2C1%2F2%5B%28n-3%29%285n%2B12%29%5D%2B18.%E7%9A%84%E5%89%8D%E6%95%B0%E5%88%97%E4%B9%8B%E5%92%8C%2C%E8%BF%99%E4%B8%80%E6%95%B0%E5%88%97%E7%9A%84%E8%A7%84%E5%BE%8B%E6%98%AF%2C%E5%AE%83%E4%BB%AC%E5%89%8D%E5%90%8E%E4%B9%8B%E5%B7%AE%E4%B8%BA%E7%AD%89%E8%AF%B7%E9%97%AE%E6%B1%82%E6%95%B0%E5%88%9718%2C34%2C55%2C81%EF%BC%8C%E2%80%A6%E2%80%A6%EF%BC%8C1%2F2%5B%28n-3%29%285n%2B12%29%5D%2B18.%E7%9A%84%E6%95%B0%E5%88%97%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B9%8B%E5%92%8C%E3%80%82%E8%BF%99%E4%B8%80%E6%95%B0%E5%88%97%E7%9A%84%E8%A7%84%E5%BE%8B%E6%98%AF%EF%BC%8C%E5%AE%83%E4%BB%AC%E5%89%8D%E5%90%8E)
请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的前数列之和,这一数列的规律是,它们前后之差为等请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的数列前n项和之和。这一数列的规律是,它们前后
请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的前数列之和,这一数列的规律是,它们前后之差为等
请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的数列前n项和之和。这一数列的规律是,它们前后之差为等差数列,
请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的前数列之和,这一数列的规律是,它们前后之差为等请问求数列18,34,55,81,……,1/2[(n-3)(5n+12)]+18.的数列前n项和之和。这一数列的规律是,它们前后
通项化简 an=1/2[(n-3)(5n+12)]+18=(5nn-3n)/2
a1=(5*1*1-3*1)/2=1
a2=(5*2*2-3*2)/2=7
a3=(5*3*3-3*3)/2=18
a1+a2+a3+……a(n-1)+an=通项之和= 先交代一下
平方和公式 1^2+2^2+……+n^2=[(n+1)^3-3n(n+1)/2-(n+1)]/3
=(n+1)(n^2+2n+1-3n/2-1)/3
=(n+1)(2n+n)/6
=n(n+1)(2n+1)/6
自然数列前n项和公式 1+2+3++……+n=n(n+1)/2
5nn=5n^2=5n(n+1)(2n+1)/6
3n =3n(n+1)/2
代入通项之和=5n(n+1)(2n+1)/12-3n(n+1)/4
化简 n(n+1)(5n-2)/6
如果数列 从第三项开始 就是 a3=18;
那么 该数列的前n项之和就是 通项之和-a2-a1=n(n+1)(5n-2)/6-8 (n从3开始)
如果n从1开始
那么b1=a3=18
那么前n项之和=(n+2)(n+3)(5n+8)/6-8 (n从1开始)
这个数列不是等差数列,也不是等比数列。前后之差为5n+1(n>=3)
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