What eight numbers can be inserted between 4 and 40 to make these ten numbers an arithmetic sequence?

（40-4)/9=4
4、8、12、16、20、24、28、32、36、40

Insert 7 numbers between 5 and 69, make them into arithmetic sequence, and find the sum of the arithmetic sequence

(5+69)*9/2=333

Insert six numbers between 8 and 36 so that they form an arithmetic sequence with the two numbers

This is simple: a total of six numbers, first find the variance:
36-8=7*d;
d=4;
Then these six numbers are 12 16 20 24 28 32

2. 5, 8.. 212. What's the average number of the strings?

212-2=210
1+210/3=71
2+212=214
214/2=107

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