an=2+20+200+……+2*10^(n-1)
=2*(10^n-1)/(10-1)
=(2/9)*(10^n-1)
所以Sn=(2/9)*[10^1+10^2+……+10^n-1-1-……-1]
=(2/9)*[10*(10^n-1)/(10-1)-n]
=(20/81)(10^n-1)-2n/9
猜你喜欢
- 1关于一路有你的作文高中800字左右急需!
- 2用a number of或the number of填空 1.I have been to Beijing_____times.
- 3I_________to go to the park this afternoon
- 4如何比较体积一样的水和花生油重量?
- 5形容"柳树"的诗句
- 6小红和妈妈今年的年龄之和是41岁,3年前妈妈比小红大25岁,妈妈和小红今年各多少岁?
- 7形容读书声音响亮的成语
- 8如图所示,轻弹簧下端固定在水平面上,一个小球从弹簧正上方某一高度处静止开始自由下落
- 9The boy didn't know___.A:what to do it B:what to do C:when to do D:how to do
- 10已知方程2x-ax=3的解是不等式5(x-2)一7