设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5+S6+15=0.
来源:学生作业帮 编辑:百度作业网作业帮 分类:数学作业 时间:2024/08/09 08:11:17
设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5+S6+15=0.
(1)若S5=5,求S6及a1;
(2)求d的取值范围.
(1)若S5=5,求S6及a1;
(2)求d的取值范围.
S5 + S6 + 15 = 0
5 + S6 + 15 = 0
S6 = -20
S5 = a1+a2+a3+a4+a5
= a1+(a1+d)+(a1+2d)+(a1+3d)+(a1+4d)
= 5*a1 + 10*d
= 5
a1 + 2d = 1
d = (1-a1)/2
S6 = a1+a2+a3+a4+a5+a6
= a1+(a1+d)+(a1+2d)+(a1+3d)+(a1+4d)+(a1+5d)
= 6*a1 + 15*d
= -20
6*a1 + 15d = -20
6*a1 + 15(1-a1)/2 = -20
12*a1 + 15 - 15*a1 = -40
55 = 3*a1
a1 = 55/3
d = (1 - 55/3)/2 = -26/3
5 + S6 + 15 = 0
S6 = -20
S5 = a1+a2+a3+a4+a5
= a1+(a1+d)+(a1+2d)+(a1+3d)+(a1+4d)
= 5*a1 + 10*d
= 5
a1 + 2d = 1
d = (1-a1)/2
S6 = a1+a2+a3+a4+a5+a6
= a1+(a1+d)+(a1+2d)+(a1+3d)+(a1+4d)+(a1+5d)
= 6*a1 + 15*d
= -20
6*a1 + 15d = -20
6*a1 + 15(1-a1)/2 = -20
12*a1 + 15 - 15*a1 = -40
55 = 3*a1
a1 = 55/3
d = (1 - 55/3)/2 = -26/3
设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5+S6+15=0.
设a1 d 为实数 首项为a1 公差为d的等差数列an前n项和为Sn 满足 S5*S6+15=0 ,若s5=5,求s6,
设a1 d 为实数 首项为a1 公差为d的等差数列an前n项和为Sn 满足 S2*S6+15=0 ,
设a1 d 为实数 首项为a1 公差为d的等差数列an前n项和为Sn 满足 S5*S6+15=0 则d的取值范围是?
设a1,d.为实数,首项为a1,公差为d的等差数列{an}的前n项和为SN,且满足S5S6 15=0及S5=5,求S6及
设a1,d为实数,首项a1,公差为d的等差数列an的前n项和为sn,满足s5s6+15=0,若s5=5求s6及a1,求d
设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5S6+15=0.
设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5S6 +15=0.1.若S5=5,求S
设a1,d为实数,首项为a1,公差为d的等差数列{an}的前n项和为Sn,满足S5S6+15=0,则d的取值范围是___
已知等差数列{an}前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列 设{bn/an}是首
若Sn为等差数列an的前n项和,S10=4*S5,d为公差,则a1/d等于
a1d为实数,首项为a1,公差为d的等差数列an的前n项和为Sn,s5s6+15=0