d =(a42-a 12)/(42- 12)=( 143-23)/30 = 4

n = 12+(an-a 12)/d = 12+(239-23)/4 = 66

2.

a 1=2.3，d=4.6，an=66.7

Which is to find n

n = 1+(an-a 1)/d = 1+(66.7-2.3)/4.6 = 15

So it took 15 second.

3.

a 1=24，a 10 & gt； 0 and a9

a 10 = a 1+9d =-24+9d & gt； 0

a9 = a 1+8d =-24+8d & lt； 0

Lian lide

24/9 & lt； d & lt24/8

that is

8/3 & lt； D< three

4.

a 1+a2+a3+a4=26

Namely:

4 a 1+( 1+2+3) d

=4a 1+6d=26 ( 1)

a2 a3=40

That is, (a1+d) (a1+2d) = 40 (2).

(1)(2) Expression of two types at the same time:

a 1=2

d=3

So these four numbers are: 2,5,8, 1 1.