SOLUTION: general term formula for 1 5 14 28 47 please help me

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Question 739620: general term formula for 1 5 14 28 47 please help me

Found 2 solutions by jim_thompson5910, MathLover1:
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
This is similar to this problem

There's a lot of work and steps. Unfortunately I don't have the time right now to show them, so here is the answer: f%28n%29+=+2.5n%5E2+-+3.5n+%2B+2 where n is a positive integer.

Note: the answer with fractions only is f%28n%29+=+expr%285%2F2%29n%5E2+-+expr%287%2F2%29n+%2B+2

Hopefully you can follow the work shown on the link I gave you. Then you can use that example to do this problem. I provided the answer as a check so you can be sure you are on the right page.

Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!
start with
n=1, 2, 3, 4, 5
to get
1 5 14 28 47 set pairs (x,y)
x|y
1|1
2|5
3|14
find a line which contains these points

y=ax%5E2%2Bbx%2Bc plug in first pair
1=a%2A1%5E2%2Bb%2A1%2Bc
1=a%2Bb%2Bc.......eq.1
plug in second pair
5=a%2A2%5E2%2Bb%2A2%2Bc
5=4a%2B2b%2Bc......eq.2
plug in third pair
14=a%2A3%5E2%2Bb%2A3%2Bc
14=9a%2B3b%2Bc......eq.3
you have a system:
1=a%2Bb%2Bc.......eq.1
5=4a%2B2b%2Bc......eq.2
14=9a%2B3b%2Bc......eq.3
______________

1=a%2Bb%2Bc.......eq.1...solve for a
1-b-c=a.....plug in eq.2
5=4%281-b-c%29%2B2b%2Bc......eq.2
5=4-4b-4c%2B2b%2Bc
5=4-2b-3c.......solve for b
2b=4-5-3c
2b=-1-3c
b=-1%2F2-3c%2F2
14=9a%2B3b%2Bc......eq.3...substitute a and b
since a=1-b-c, substitute b
a=1-%28-1%2F2-3c%2F2%29-c
a=1%2B1%2F2%2B3c%2F2-c
a=3%2F2%2B3c%2F2-2c%2F2
a=3%2F2%2Bc%2F2
14=9%283%2F2%2Bc%2F2%29%2B3%28-1%2F2-3c%2F2%29%2Bc.......solve for c
14=27%2F2%2B9c%2F2-3%2F2-9c%2F2%2Bc
14=24%2F2%2Bc
14=12%2Bc
14-12=c
c=2
now find a
a=3%2F2%2Bc%2F2
a=3%2F2%2B2%2F2
a=5%2F2
and find b
b=-1%2F2-3c%2F2
b=-1%2F2-3%2A2%2F2
b=-1%2F2-6%2F2
b=-7%2F2
so,
a=5%2F2
b=-7%2F2
c=2
since we are looking for general term a%5Bn%5D, set y=a%5Bn%5D
a%5Bn%5D=%285%2F2%29x%5E2%2B%287%2F2%29x%2B2
a%5Bn%5D=%281%2F2%29%285x%5E2%2B7x%2B2%2A2%29
highlight%28a%5Bn%5D=%281%2F2%29+%285n%5E2-7n%2B4%29%29....your general term formula where n=1,2,3.......

check: n=1
a%5B1%5D=%281%2F2%29+%285%2A1%5E2-7%2A1%2B4%29
a%5B1%5D=%281%2F2%29+%285-7%2B4%29
a%5B1%5D=%281%2F2%29+%282%29
a%5B1%5D=1

n=2
a%5B2%5D=%281%2F2%29+%285%2A2%5E2-7%2A2%2B4%29
a%5B2%5D=%281%2F2%29+%2820-14%2B4%29
a%5B2%5D=%281%2F2%29+%2810%29
a%5B2%5D=5
and so on.......