1.在等差数列{an}中,已知a1=-1,an=-5,sn=-27,求n和d2.在等差数列{an}中,a10=230,a25=-220,求n为何值时,Sn的值最大3.已知等差数列{an}满足a1=2,a3=4,bn=an-1,若cn=an+bn,求cn的前10项和
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![1.在等差数列{an}中,已知a1=-1,an=-5,sn=-27,求n和d2.在等差数列{an}中,a10=230,a25=-220,求n为何值时,Sn的值最大3.已知等差数列{an}满足a1=2,a3=4,bn=an-1,若cn=an+bn,求cn的前10项和](/uploads/image/z/8405810-26-0.jpg?t=1.%E5%9C%A8%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E4%B8%AD%2C%E5%B7%B2%E7%9F%A5a1%3D-1%2Can%3D-5%2Csn%3D-27%2C%E6%B1%82n%E5%92%8Cd2.%E5%9C%A8%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E4%B8%AD%2Ca10%3D230%2Ca25%3D-220%2C%E6%B1%82n%E4%B8%BA%E4%BD%95%E5%80%BC%E6%97%B6%2CSn%E7%9A%84%E5%80%BC%E6%9C%80%E5%A4%A73.%E5%B7%B2%E7%9F%A5%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E6%BB%A1%E8%B6%B3a1%3D2%2Ca3%3D4%2Cbn%3Dan-1%2C%E8%8B%A5cn%3Dan%2Bbn%2C%E6%B1%82cn%E7%9A%84%E5%89%8D10%E9%A1%B9%E5%92%8C)
1.在等差数列{an}中,已知a1=-1,an=-5,sn=-27,求n和d2.在等差数列{an}中,a10=230,a25=-220,求n为何值时,Sn的值最大3.已知等差数列{an}满足a1=2,a3=4,bn=an-1,若cn=an+bn,求cn的前10项和
1.在等差数列{an}中,已知a1=-1,an=-5,sn=-27,求n和d
2.在等差数列{an}中,a10=230,a25=-220,求n为何值时,Sn的值最大
3.已知等差数列{an}满足a1=2,a3=4,bn=an-1,若cn=an+bn,求cn的前10项和
1.在等差数列{an}中,已知a1=-1,an=-5,sn=-27,求n和d2.在等差数列{an}中,a10=230,a25=-220,求n为何值时,Sn的值最大3.已知等差数列{an}满足a1=2,a3=4,bn=an-1,若cn=an+bn,求cn的前10项和
1)Sn=(a1+an)*n/2
=(-1-5)*n/2
= -27
得 n=9
an=a1+(n-1)*d
= -1+(9-1)*d
= -5
得 d= -1/2
2)a25 -a10 = 15*d
-220 -230=15*d
得 d= -30
an= a1+(n-1)*d
=230 -30*(n-1)
令an<0,即230 -30*(n-1)<0
解得 n>8又2/3
所以,当n=8时,Sn最大.【n≤8时,各项都是正数;n≥9后出现负数,Sn变小】
3)a3 -a1= 4 -2 =2 =2d
则 d=1
cn=an+bn
=an+an-1
=2an -1
c(n-1) =2a(n-1) -1
cn - c(n-1) =2an -2a(n-1)
=2d
=2
所以,{cn}是公差为2的等差数列,首项c1=2a1-1= 3,c10=3+9*2=21
S10=(c1+c10)*10/2
=(3+21)*10/2
= 120
1)an=a1+4d sn=(a1+an)/d*n
-5=-1+4d -27=(-1-5)/-1*n
d=-1 n=-2/9