數列(急!!!!!!!!!!)

We can find the relationship between the terms of the sequence:
2nd term-1st term=3-1=2
3rd term-2nd term=6-3=3
4th term-3th term=10-6=4
Therefore 5th term-4th term=5th term-10=5
Therefore 5th term=10+5=15
The 1st term=1
The 2nd term=1+2=3
The 3rd term=1+2+3=6
The 4th term=1+2+3+4=10
The n th term=1+2+3+.....+n=n(n+1)/2
Therefore the general term is n(n+1)/2

1, 3, 6, 10
1st term = 1
2nd term = 1+2 = 3
3rd term = 3+3 = 6
4th term = 6+4 = 10
5th term = 10+5 =15
6th term = 15+6 = 21
:
:
n term = (n-1term)+n

Therefore, the 5 th term is 15.

1,3,6,10,15
1+2=3
3+3=6
6+4=10
10+5=15

5th term =15
general term=1+2+3+4+...+n=n(n+1)/2