1. How to find the nth term of the sequence of 3, 6, 10, 15 and 21?

1. How to find the nth term of the sequence of 3, 6, 10, 15 and 21?

Add up to find the general term: a (n + 1) - an = n + 1, an - A (n-1) = n.. A2-a1 = 2. Add all the formulas left and right to get the general term, an = (n ^ 2 + n) / 2. Let's put n ^ 2 together to find n and put n aside to find Sn = [(1 ^ 2 + 2 ^ 2 +. + n ^ 2) + (1 + 2 + 3 +... + n)] / 2. Let's do 1 ^ 2 + 2 ^ 2 +. + n ^ 2

What is the 100th item of the known sequence 3 9 15 21 27 、

a100 = a1+(n-1)d = 3+(100-1)*6=597

Excel even and odd line separate summation formula
Example: Line A1 is 123, line A2 is 321, line A3 is 456, line A4 is 789
I added a1 + a3 to get the sum
And the sum of A2 + A4
Because I want a1 + a3 to be added to A601 all the time, I use a very stupid way to add them one by one like this, so excel says that the formula is too complex to add, so I can only add them on my own calculator
Ask you to see if there is a simple formula for even and odd numbers to sum separately

=sumproduct(mod(row(a1:a601),2)*a1:a601)
=sumproduct((1-mod(row(a1:a601),2))*a1:a601)