Why is it expressed as 3n-2 in sequence 1,4,7

Tolerance d = 3, first term = 1; according to the general term formula a of arithmetic sequence_ n=a_ 1 + (n-1) d for a_ N = 1 + (n-1) * 3

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1. If the coefficient of an unknown is 1 / 2 and the solution of the equation is 3, then the equation of the unknown can be written as
2. In the triangle ABC, if a = x cm, B = 2 cm and ∠ B = 45 degree sub triangle have two solutions, what is the value range of X? Please bsq20080408 give the process,
3. In △ ABC, a = x, B = 2, B = 45 °, if the triangle has two solutions, then the value range of X is () A. x＞2B. x＜2C. 2＜x＜22D. 2＜C＜23
4. In the triangle ABC, given B = 45 degrees, B = 2, find a value range : A is greater than 0 and less than or equal to 2 times the root sign 2
5. In the known triangle ABC, BC = x, AC = 2, B = 45 degrees, if the triangle has two solutions, then the value range of X is?
6. In △ ABC, the corresponding sides of angles a, B and C are x, B and C respectively. If △ ABC with B = 2 and B = 45 ° has exactly two solutions, then the value range of X is () A. （2，+∞）B. （0，2）C. (2，22)D. (2，2)
7. If the angle a is 45 degrees and the length of the center line on the BC side is 3, the maximum area of the triangle ABC is obtained
8. In the triangle ABC, if a = x, B = 2, angle B = 45 degrees, if there are two triangle ABC, then x range RT
9. In △ ABC, a = x, B = 2, B = 45 °, if the triangle has two solutions, then the value range of X is () A. x＞2B. x＜2C. 2＜x＜22D. 2＜C＜23
10. It is known that in the triangle ABC, the angle B is equal to twice the angle c, and the proof of AC = AB + BD is given
11. For any positive integer n, is the value of integral (3N + 1) (3n-1) - (3-N) (3 + n) a multiple of 10? If it is a multiple of 10, explain the reason
12. 2/1*4+2/4*7+2/7*10+2/10*13+.+2/(3n-2)(3n+1)
13. How many terms are there in 1,4,7,10.3n + 10?
14. How to calculate the number of items? 1 + 4 + 10 +. + (3N + 4) + (3N + 7) =? How many items in total?
15. 1+4+7+10+.+（3n+4)+(3N+7)=
16. Given that f (x) is defined on a set of positive integers, for any n belonging to a positive integer, f (f (n)) = 3N + 2, f (2) = 1, then f (80) =?
17. Function f (x). For any n belonging to a positive integer, f (f (n)) = 3N, f (n + 1) > F (n), f (n) belongs to a positive integer. Find f (1) and f (12)
18. It is known that the function f (x) is a monotone increasing function on (0, positive infinity). When n belongs to a positive integer, f (n) belongs to a positive integer, and f (f (n)) = 3N, then f (5) =
19. If the function f (x) = x ^ (n ^ 2-3n) (M belongs to Z) is even and monotone decreasing on (0, + ∞), then n =,
20. It is known that f (x) is an increasing function defined on (0, + ∞). When n ∈ n *, f (n) ∈ n *, f [f (n)] = 3N, then f (1) + F (2)=______ ．