Sequence 1.3.9.15.21. The nth number is

(n+1)n/2

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3. How to find the sum of reciprocal squares of natural numbers?
4. Ask for the sum of the reciprocal sequence of natural numbers, 1+1/2+1/3+1/4+ … +Sum of reciprocal sequence of 1 / n natural numbers And 1 + 1 / (2 * 2) + 1 / (3 * 3) + 1 / (4 * 4) + +Sum of reciprocal square sequence of 1 / (n * n) natural number ～ Many people say it can't be solved Another point I ask is the sum of the first n terms, not the limit. But others say it's →∞ I don't understand why the sequence 2481632... 2yn is also →∞, but the sum of the first n terms can be obtained?
5. The square of X + 4xy + 4Y is equal to? (factorization)
6. Hello: how about X (X-Y) ^ 2-y (Y-X) ^ 2 x ^ 2 / 4 + XY + y ^ 2 using the square difference formula?
7. Can the square of x plus 2, the square of XY plus y be calculated by the complete square formula
8. (1, 2, 3, 4. N) how many logarithms of vertex angle are there when the lines intersect at a point? What is the law? Online, etc
9. Law of logarithm: straight lines intersect at one point, two intersect, there are two pairs of logarithm of vertex angle, and six pairs of logarithm of adjacent complementary angle Straight lines intersect at one point and two intersect. There are two pairs of logarithms of vertex angles and six pairs of logarithms of adjacent complementary angles And so on, how many pairs of vertex angles do n lines intersect
10. The logarithm of the vertex angle formed by the intersection of three different lines on the plane is
11. In sequence 1.4.9.16.25.36 What's the 79th number in In sequence 1.4.9.16.25.36 What's the 79th number in? The fourth grade of primary school
12. What is the law of sequence 1,4,9,25
13. The sum of the first n terms of sequence 3 / 2,9 / 4,13 / 4,65 / 16 is___
14. The sum of the first n terms of sequence 1 / 29 / 4 25 / 8 65 / 16
15. The first four terms of sequence {an} are 3 / 2,9 / 4,25 / 8,65 / 16. Try to find the general term formula and the sum of the first n terms
16. Sequence {1,1 / 4,1 / 9 , 1 / N ^ 2}, how to sum
17. Sum 0.9 + 0.99 + 0.999 + 0.9999 + +0.99999…… 9 (n 9S) this is in the sequence
18. K2 = λ + 1 / λ - 2,4 ≤ λ ≤ 9.k2? Find the specific process
19. How to sum arithmetic sequence
20. Asking for help -- the problem of summation of arithmetic sequence Given that the tolerance of the arithmetic sequence {an} is 1, and a1 + A2 +... + A98 + A99 = 99, then the value of A3 + A6 + A9 +... + A99 is_____