# Homework Help: How do you find the equation of a Cubic function given 5 points? (no zeros)?

1. Jan 8, 2012

### srizen

1. The problem statement, all variables and given/known data

what the questions asks is that i need to find the equation of a polynomial with these given points:
1,1
2,-3
3,5
4,37
5,105

i know that one way to solve is by creating 5 equations then solve for ax^3+bx^2+cx+d using the elimination/substitution method.
however is there another, much easier way of doing this question?

2. Relevant equations

ax^3+bx^2+cx+d

3. The attempt at a solution

1= a+b+c+d
-3=8a+4b+2c+d
5= 27a+9b+3c+d
37=64a+16b+4c+d
108=125a+25b+5c+d

fixed, yes, my mistake

Last edited: Jan 8, 2012
2. Jan 8, 2012

### eumyang

Three of the equations are wrong. They should be
1= a+b+c+d
-3=8a+4b+2c+d
5= 27a+9b+3c+d
37=64a+16b+4c+d
108=125a+25b+5c+d

Also, you don't need the last equation, because there are 4 unknowns.

As for other methods, there's the finite difference method, but I don't think it will help for this particular problem (because you already told us that this is a cubic).

Last edited: Jan 8, 2012
3. Jan 8, 2012

### srizen

fixed, i typed the equations too fast

4. Jan 8, 2012

### SammyS

Staff Emeritus
True, but I got the coefficients fairly quickly using a difference method.

Actually, after playing around a bit with this, I got the result with two different difference methods.

5. Jan 8, 2012

### srizen

what exactly is the difference method?
i ask because i've tried this question, and i kept getting it wrong.

6. Jan 8, 2012

7. Jan 8, 2012

### SammyS

Staff Emeritus
Make a table of differences

$$\begin{array}{|c|c|c|c|c|c|c|c|c|} \hline \quad i \quad & \quad x_i \quad & \quad f(x_i) \quad &\quad (\Delta^1)_i \quad &\quad (\Delta^2)_i \quad & \quad (\Delta^3)_i \quad & \quad (\Delta^4)_i \quad \\ \hline & & & & & & \\ 1 & 1 & 1 & -4 & 12 & 12 & 0 \\ & & & & & & \\ 2 & 2 & -3 & 8 & 24 & & -- \\ & & & & & & \\ 3 & 3 & 5 & & & -- & -- \\ & & & & & & \\ 4 & 4 & 37 & & -- & -- & -- \\ & & & & & & \\ 5 &5 & 105 & -- & -- & -- & -- \\ & & & & & & \\ \hline \end{array}$$

Where: $(\Delta^1)_i=f(x_{i+1})-f(x_{i})\,,$

$(\Delta^2)_i=(\Delta^1)_{i+1}-(\Delta^1)_i$

etc.

See if you can fill in the rest.

If f(x) is truly a cubic function then the Δ3 column will all be the same.

Fill out a similar Table for g(x) = x3 . The Δ3 column will all be 6's.

What do you suppose that means about the x3 coefficient of f(x) ?

8. Jan 9, 2012

### srizen

OMG! i love you forever, i had no idea this method existed, i already solved through almost an hour of writing matrices, with this i solved it in 3 minutes. thank you!

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