# Finding equation of a cubic function?

• Mar 20th 2010, 02:56 PM
llamajuice
Finding equation of a cubic function?
How do you do find the equation of a cubic function?
• Mar 20th 2010, 03:02 PM
skeeter
Quote:

Originally Posted by llamajuice
How do you do find the equation of a cubic function?

find what ... zeros?

http://www.mathhelpforum.com/math-he...functions.html
• Mar 20th 2010, 03:10 PM
llamajuice
theres no zeroes.
like its just a normal graph and i need to find an equation that models the behavior of the graph.
• Mar 20th 2010, 03:23 PM
skeeter
Quote:

Originally Posted by llamajuice
theres no zeroes.
like its just a normal graph and i need to find an equation that models the behavior of the graph.

... how do you know the data models a cubic equation?
• Mar 20th 2010, 03:28 PM
llamajuice
because it looks like one. :D
well first i just plotted the points into excel then i tried all the lines of best fit to see which one it matched. It could also be a sine graph thing, but any polynomial of a degree of 3 or higher works too so i thought finding a cubic function that matches it would be easiest.
do i just sub in points into y=ax^3 +bx^2 +cx +d to get 4 formulas then solve?
• Mar 20th 2010, 03:37 PM
skeeter
Quote:

Originally Posted by llamajuice
because it looks like one. :D
well first i just plotted the points into excel then i tried all the lines of best fit to see which one it matched. It could also be a sine graph thing, but any polynomial of a degree of 3 or higher works too so i thought finding a cubic function that matches it would be easiest.
do i just sub in points into y=ax^3 +bx^2 +cx +d to get 4 formulas then solve?

give it a go ... I cannot make a judgement w/o knowing how the data originated.
• Mar 20th 2010, 03:47 PM
llamajuice
its like height vs age so theres no x- intercepts
• Mar 20th 2010, 03:48 PM
e^(i*pi)
Quote:

Originally Posted by llamajuice
its like height vs age so theres no x- intercepts

Height vs age is likely to be linear since there's a direct correlation. Best thing to do is plot in excel and guess the best fit
• Mar 20th 2010, 03:59 PM
llamajuice
scratch that its body mass index