Observe the sequence of numbers: 1, 3, 6, 10, 15, 21... According to the sequence of numbers in this column, what is the nth number?

an=n*(n+1)/2

RELATED INFORMATIONS

1. 1, - 3,5, - 7,9, - 11... In this sequence, if the nth number is a, what is the nth + 1 number?
2. Write the general term formula of sequence 1 / 2,1 / 4, - 5 / 8,13 / 16, - 29 / 32,61 / 64
3. Mathematical problems: write the general formula of this sequence: 1 / 2,1 / 4, - 5 / 8,13 / 16, - 29 / 32,61 / 64
4. Sequence 1 / 2, 1 / 4, - 5 / 8, 13 / 16, - 29 / 32, 61 / 64
5. Find sequence 1 / 2,1 / 4, - 5 / 8,13 / 16, - 29 / 32,61 / 64 General term of
6. Sequence 1,5,13,29,61 A general formula of is
7. Write the rule of rabbit in Java as sequence 1,1,2,3,5,8,13,21 There is a pair of rabbits, every month from the third month after birth They all give birth to a pair of rabbits. When the rabbits grow up to the third month, they give birth to another pair of rabbits every month, If rabbits do not die, how many rabbits are there in each month?
8. In the rabbit sequence 1,1,2,3,5,8,13,21 What is the remainder of item 220 divided by 3?
9. Rabbit Series
10. Rabbit sequence 1,2,3,5,8 Each number is the sum of its first two terms. What is the number 1991?
11. Observe the number of columns 3, 8, 15, 24, 35, 48 By analogy, the smallest integer larger than 2014 in this sequence is
12. Observe a column number, 3, 8, 15, 24, 35, 48 And so on, what is the smallest integer larger than 2010 in this series?
13. The number of one column is - (1), 8 / 5, - (15 / 7), 24 / 9, - (35 / 11), 48 / 13 What is the twentieth term in this sequence?
14. -1、8/5、-17/5、24/9、-35/11、48/13…… What is the tenth term in the sequence?
15. Give a sequence how to quickly know its general formula, is there a rule I feel that it needs more feeling than painting color, that is, to make up the number. Although it can be made, it always takes some time. Why?
16. General term formula of sequence 2.5.2.5
17. 5,0,5,0,5, what is the general formula of this series? What is the general term formula of 5,0,5,0,5,0?
18. Write a general formula of the following sequence: 1, 0, - 13, 0, 15, 0, - 17, 0
19. Observe the characteristics of the following sequence, fill in the blanks with appropriate numbers, and write a general formula for each sequence 1.（）,-4,9,（）,25,（）,49 1, radical 2, (), 2, radical 5, (), radical 7 ------------------ How to write this,
20. Find the rule and write the general formula of each sequence * 3 1,2/3,1/2,2/5,1/3,( ),1/4,... 3/2,1,5/8,3/8,7/32,... 1,3,7,15,31,..