general term 1,2,4,7,11

1st term: 1=1

2nd term: 2=1+1

3rd term: 4=1+1+2

4th term: 7=1+1+2+3

5th term: 11=1+1+2+3+4

......

∴nth term= 1+[1+2+3+...+(n-1)]

=1+n(n-1)/2

1st term: 1=1

2nd term: 2=1+1

3rd term: 4=1+1+2

4th term: 7=1+1+2+3

5th term: 11=1+1+2+3+4

......

∴nth term= 1+[1+2+3+...+(n-1)]

=1+n(n-1)/2

value of nth term=1+[(n-1)(n)/2]

1st term=[1+(1-1)1/2]=1
2nd term=[1+(2-1)2/2]=2
：
：
：
nth term=1+[(n-1)n/2]
至於why,咪就係睇下每個term之間的diff.