There is a column of 2.6.10.14.18. What's the 50th number

There is a column of 2.6.10.14.18. What's the 50th number

The tolerance of arithmetic sequence is 4
The number 50 is 198

There is such a column of numbers: 3, 7, 12, 18, 25. What is the 10th number and the nth number

Let {an} be an arithmetic sequence, then a2-a1 = 4a3-a2 = 5a4-a3 = 6.. an-a (n-1) = n + 2 add all the two sides of the above formula, then an-a1 = 4 + 5 + 6 +... + (n + 2) can be obtained. If this is an arithmetic sequence, then an-a1 = (n ^ 2 + 5n-6) / 2 and A1 = 3, so an = (n ^ 2 + 5n-6) / 2 + 3 = (n ^ 2 + 5N) / 2 band