# Math help: exponential function

Discussion in 'Off Topic [BG]' started by Figjam, Sep 10, 2005.

1. ### Figjam

Aug 5, 2003
Boston, MA
Given 3 points: A(1,20) B(5,10) C(9,5)
Create an exponential function thru points A, B, C. Find the equation with y in terms of x. ...the formula that was given to do this is (y= a * b (x+c(/d))

Apr 13, 2001
berkeley, ca
should that "*" be a "^"?

what you do is make three equations, given those data points.

you have:

x_1 = 1, y_1 = 20,
x_2 = 5, y_2 = 10
x_3= 9, y_3 = 5

plug those into the general equation.

now you have a system of three equations--you can now solve for three constants!

...trouble is... looks like you have four constants in there.

3. ### Figjam

Aug 5, 2003
Boston, MA
Its a multiplication sign.

whats the underscore?

The end goal is for one equation to graph it.

Apr 13, 2001
berkeley, ca

umm...

i think you mean

y = a*b^[(x+c)/d] ...?

but there's one letter too many.

5. ### Figjam

Aug 5, 2003
Boston, MA
Sorry it was typed wrong originally,you did it right.

Im as confused as you are on the 4th thing though, but one equation is somehow needed in the end.

Apr 13, 2001
berkeley, ca
i used the underscore to denote a particular value of x with a corresponding particular value of y.

replace "1,2,3" with "A,B,C." then it would match up better and make more sense, maybe, since A, B, and C are what the points are called.

Apr 13, 2001
berkeley, ca

hmm... i'm going to work on this one. (just finished my own hw. )

Apr 13, 2001
berkeley, ca
well, i made a graph on excel, and it definitely looks exponential!

it decayed (that is, got smaller as x increased).

anyway, a trick that you might use is:

b^(a+c) = b^a * b^c

--> i think this'll eliminate one of our constants; it just gets folded into our coefficient.

so my recommendation is to set c=0 in the equation you want to solve for.

that and a lot of algebra ought to do it.