How to find the number of items in a sequence? For example, how to find the number of items in a sequence an = (2n + 1) + (2n-1) + (2n-2) +. + 7 + 5 + 1 There is also a general way to find the number of items in a sequence I'm in the third year of senior high school now. I'm going through the first round of review to find out how to learn
You make a lot of mistakes
In arithmetic sequence
an=a1+(n-1)d
So n = (an-a1) / D + 1
Here d = 2
The maximum is 2n + 1 and the minimum is 1
So the number of terms = [(2n + 1) - 1] / 2 + 1 = n + 1
If it's equal
an=a1q^(n-1)
So Q ^ (n-1) = an / A1
n-1=logq(an/a1)
n=logq(an/a1)+1