{an}为等差数列,a1+a3+a5=105,a2+a4+a6=99
∴3a1+6d=105
3a1+9d=99
∴d=-2,a1=39
∴an=39-2(n-1)=41-2n
Sn=(39+41-2n)*n/2=(20-n)n
a20=1
{an}为等差数列,a1+a3+a5=105,a2+a4+a6=99,求an与sn及a20的值
{an}为等差数列,a1+a3+a5=105,a2+a4+a6=99,求an与sn及a20的值
数学人气:603 ℃时间:2019-10-19 17:41:44
优质解答
我来回答
类似推荐
猜你喜欢
- 1数学趣味故事及解析
- 2—Would you like something to drink?—________.A.yes ,I would B.OK ,I am thirsty C.Yes ,please
- 31·Tom often does his homework in the evening(用now改写句子)Tom ------- ------- his homework now
- 4铝和氨水反应生成什么?
- 5根号10 +根号14+根号15+根号21分之根号5加根号7
- 6已知a(平方)+b(平方)+2a-4b+5=0,求2a(平方)+4b-3的值.
- 71.I 'm going to find a part-time job for a year or two I'm going to find a part-time job for_ _ _
- 8Tom___ China for 3 years.A.has been B.has been in C.has been to D.has been at
- 976.5平方米等于多少平方分米?
- 10第一根绳子长二分之一米,第二根绳子比第一棍长二分之一,第二根绳子长多少米?