Posted by Cassie on .
The pattern is:
4, 8, 15, 25
Is the formula:
2.5n + 1.5n???
If you are wondering what the standard formula is, check my previous question.
Thank you! :)

Algebra 
Reiny,
your formula can't be right,
try n=3 > 2.5(3) + 1.5(3) = 12 not the 15 that are needed.
I do these the following way:
4,8,15,25
the first set of differences between successive terms is 4,7,10
and the second differences are 3,3
so we have a constant difference in the second set, so the function must be quadratic
let it be y = an^2 + bn + c
from you data:
4 = a+b+c
8 = 4a+2b+c
15 = 9a+3b+c
subtract the first two: 4 = 3a + b
subtract the last two : 7 = 5a + b
now subract these two 3 = 2a
a = 3/2
sub into 4 = 3a+b > b = 1/2
and finally back in the first gives us
c = 3
so y = (3/2)n^2  n/2 + 3
try it, it works.