Number 2, 7, 15, 26, 40. (n) what is the law, find out the algebraic formula

Number 2, 7, 15, 26, 40. (n) what is the law, find out the algebraic formula

a1=2
a2=7
a3=15
…
a2-a1=5
a3-a2=8
a4-a3=11
……
8-5=11-8=14-11=…… =3
introduction
an=a1+ [10+3（n-2)](n-1)/2 =3/2(n^2)+(1/2)n

3 / 4, 9 / 8, 15 / 16, 21 / 24, () fill in the regular number
3 / 4, 9 / 8, 15 / 16, 21 / 24, () fill in the regular number

The answer is 27 / 36
The data is divided into numerator and denominator;
The molecules are 3, 9, 15 and 21, and the rule is f (n) = 3 * (2n-1)
The denominator is 4,8,16,24, and the rule is f (n) = f (n-2) + F (n-1) + 4 * (4-N)

Fill in 6 7 9 12 16 〔 2 3 5 8 13 21 〕

6 7 9 12 16 【21】
6+1=7
7+2=9
9+3=12
12+4=16
16+5=21
2 3 5 8 13 21 【 34】
2+3=5
3+5=8
5+8=13
8+13=21
13+21=34