等差数列{an}中,a10=23,a25=-22. (1)求an (2)求n使Sn最大 (3)Tn=│a1│+│a2│+...+│an│.求Tn速度!
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等差数列{an}中,a10=23,a25=-22. (1)求an (2)求n使Sn最大 (3)Tn=│a1│+│a2│+...+│an│.求Tn速度!
等差数列{an}中,a10=23,a25=-22. (1)求an (2)求n使Sn最大 (3)Tn=│a1│+│a2│+...+│an│.求Tn
速度!
等差数列{an}中,a10=23,a25=-22. (1)求an (2)求n使Sn最大 (3)Tn=│a1│+│a2│+...+│an│.求Tn速度!
1) 15d=-45 d=-3 a1=50
an=50-3(n-1)=53-3n
2) Sn=n(a1+an)/2 =n(103-3n)/2=(-3n^2+103n)/2
有n=-b/2a=17
3)有a18=-1
则有Tn=S17-(Sn-S17)=2S17-Sn=(3/2)*n^2-(103/2)n+884
a25=a10+15d
-22=23+15d
15d=-45
d=-3
a10=a1+9d
23=a1+9*(-3)
a1=50
an=a1+(n-1)d
=50-3(n-1)
=53-3n
an>=0
53-3n>=0
3n<=53
n<=53/3
即n=17时,an>0
所以...
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a25=a10+15d
-22=23+15d
15d=-45
d=-3
a10=a1+9d
23=a1+9*(-3)
a1=50
an=a1+(n-1)d
=50-3(n-1)
=53-3n
an>=0
53-3n>=0
3n<=53
n<=53/3
即n=17时,an>0
所以此时Sn的值最大
a18=53-18*3=-1
Tn=│a1│+│a2│+...+│an│
=a1+a2+.........+a17+│a18+...+an│
=(2a1+16d)*17/2+│(-1+53-3n)*(n-17)/2│
=17(a1+8d)+│(52-3n)*(n-17)/2│
=17*(50-3*8)+│(52-3n)*(n-17)/2│
=17*26+(3n-52)*(n-17)/2
=442+(3n^2-103n+884)/2
=(3n^2-103n+1768)/2
收起
1.15d=a25-a10= -45 d= -3 a1=50 an=53-3n
2.an=53-3n>0时 n≦17.n=17
3.Tn=17(a1+a17)/2-(n-17)(a18+an)/2