Posted by **Cassie** on Monday, April 20, 2009 at 4:07pm.

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**, Monday, April 20, 2009 at 5:03pm
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.

## Answer this Question

## Related Questions

- Algebra - The pattern is: 4, 8, 15, 25 Is the formula: 2.5n + 1.5n??? If you are...
- Pre-Algebra - Please help to solve to find x,y 1. y=7x-3 2. 4x+2y=8 Take the ...
- pre algebra - write an equation for the line through the given points or through...
- math30 - A sequence has a first term of 24 and every other term is one half of ...
- Chemistry - What is the empirical formula for aluminium selenide? What is the ...
- Algebra - Solve by using the quadratic formula. 3x^2=11x+4. Look up the formula ...
- algebra - What is the slope of a line through (-5,2) and (6,7) in the standard(x...
- algebra - how would i work out the following problems solve each formula in ...
- algebra 2 - Can you find the error for me in this equation, find the explicit ...
- integrated algebra - how can you solve x^2 - 3x = 0 i was using a formula as: x^...