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.