有点难的数列(在线等)1.若x不等于y.数列x a1 a2 y 和x b1 b2 b3 y各自都成等差数列,则a2-a1除于b2-b1=?2.若等差数列{an}各项都是正,且a3a5+a3a8+a5a10+a8a10=64 则S12=____3.数列{an}中a1=8 a4=2且a(n+2)-2a(n+1)+an=0
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![有点难的数列(在线等)1.若x不等于y.数列x a1 a2 y 和x b1 b2 b3 y各自都成等差数列,则a2-a1除于b2-b1=?2.若等差数列{an}各项都是正,且a3a5+a3a8+a5a10+a8a10=64 则S12=____3.数列{an}中a1=8 a4=2且a(n+2)-2a(n+1)+an=0](/uploads/image/z/1159741-37-1.jpg?t=%E6%9C%89%E7%82%B9%E9%9A%BE%E7%9A%84%E6%95%B0%E5%88%97%EF%BC%88%E5%9C%A8%E7%BA%BF%E7%AD%89%EF%BC%891.%E8%8B%A5x%E4%B8%8D%E7%AD%89%E4%BA%8Ey.%E6%95%B0%E5%88%97x+a1+a2+y+%E5%92%8Cx+b1+b2+b3+y%E5%90%84%E8%87%AA%E9%83%BD%E6%88%90%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2C%E5%88%99a2-a1%E9%99%A4%E4%BA%8Eb2-b1%3D%3F2.%E8%8B%A5%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E5%90%84%E9%A1%B9%E9%83%BD%E6%98%AF%E6%AD%A3%2C%E4%B8%94a3a5%2Ba3a8%2Ba5a10%2Ba8a10%3D64+%E5%88%99S12%3D____3.%E6%95%B0%E5%88%97%7Ban%7D%E4%B8%ADa1%3D8+a4%3D2%E4%B8%94a%28n%2B2%29-2a%28n%2B1%29%2Ban%3D0)
有点难的数列(在线等)1.若x不等于y.数列x a1 a2 y 和x b1 b2 b3 y各自都成等差数列,则a2-a1除于b2-b1=?2.若等差数列{an}各项都是正,且a3a5+a3a8+a5a10+a8a10=64 则S12=____3.数列{an}中a1=8 a4=2且a(n+2)-2a(n+1)+an=0
有点难的数列(在线等)
1.若x不等于y.数列x a1 a2 y 和x b1 b2 b3 y各自都成等差数列,则
a2-a1除于b2-b1=?
2.若等差数列{an}各项都是正,且a3a5+a3a8+a5a10+a8a10=64 则S12=____
3.数列{an}中a1=8 a4=2且a(n+2)-2a(n+1)+an=0 则an的通项公式______
Sn=|a1|+|a2|+|a3|…|an| 求Sn
有点难的数列(在线等)1.若x不等于y.数列x a1 a2 y 和x b1 b2 b3 y各自都成等差数列,则a2-a1除于b2-b1=?2.若等差数列{an}各项都是正,且a3a5+a3a8+a5a10+a8a10=64 则S12=____3.数列{an}中a1=8 a4=2且a(n+2)-2a(n+1)+an=0
1.a2-a1=(y-x)/3 b2-b1=(y-x)/4
所以(a2-a1)/(b2-b1)=4/3
2.由题意:(a5+a8)*(a3+a10)=64
a5+a8=a3+a10所以a5+a8=8(an>0)
s12=(a1+a12)*12/2=(a5+a8)*12/2=8*6=48
3.原式即:
a(n+2)-a(n+1)=a(n+1)-a(n)
an必然是等差数列,设公差为d
又因为a4-a1=(a4-a3)+(a3-a2)+(a2-a1)=3d
而a4-a1=-6 所以d=-2
又a1=8
所以an=10-2n
1
a2-a1除于b2-b1=(y-x)/3/(y-x)/4=4/3
2
a3(a5+a8)+a10(a5+a8)=a3(a3+a10)+a10(a3+a10)=64
a3+a10=8
s12=12*8/2=48