# Quadratic function and exponential function question!

• Mar 30th 2008, 11:23 AM
bubblegum
Quadratic function and exponential function question!
hi, it's me again. Thanks so much for the help last time. :)

i have this table of values:
x | y
1 | 20
3 | 320

i have to make a linear, quadratic, and exponential function from these values.
i used y-y1=m(x-x1) for the linear and came up with f(x)= 150x-130

am i supposed to use the quadratic formula to find the quadratic function? i tried but I ended up being unable to find an answer (undefined?). can someone please walk me through this? maybe I just need the values for a, b, and c.

as for the exponential function, i am completely clueless... (Doh)

as always, i am not allowed to use a calculator for these problems, so i hope someone can guide me through the steps.(Nod)
• Mar 30th 2008, 11:38 AM
galactus
Do you have another set of data points for the quadratic?.

As it is, you have

$\displaystyle a+b+c=20$

$\displaystyle 9a+3b+c=320$

We need one more because we have three variables but only two equations.

Unless, we can assume c=0 and have $\displaystyle y=ax^{2}+bx$

Then we have

$\displaystyle a+b=20$

$\displaystyle 9a+3b=320$

Then we solve the system and get

$\displaystyle a=\frac{130}{3}, \;\ b=\frac{-70}{3}$

For the exponent, we can use $\displaystyle y=a\cdot{b^{t}}$

$\displaystyle 20=a\cdot{b^{1}}$

$\displaystyle 320=a\cdot{b^{3}}$

Solve for a and b.
• Mar 30th 2008, 02:28 PM
bubblegum
thank you!!!

a problem with the quadratic one... how do i figure out how f(x)=170?
if i do 170= 130x^2 + -20/3x, what are the next steps?

for the exponential one... i came up with a=88.657... probably not right, cuz i don't think it makes sense. help please?
also, how do i find f(x+3)?
sorry for all these questions. (Doh)