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)