Intersections of a line and a curve

1. Jul 21, 2007

ehrenfest

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

For all all lines which meet the graph y = 2x^4+7x^3+3x-5 at four distinct points, what is the sum of the x-coordinates of the four points of intersection?

2. Relevant equations

3. The attempt at a solution

So, you obviously set ax+b =2x^4+7x^3+3x-5, but how do you find the zero's of that?

Last edited: Jul 21, 2007
2. Jul 21, 2007

Hurkyl

Staff Emeritus
You don't need to know the roots; you only need to know their sum.

3. Jul 22, 2007

ehrenfest

Yes and how do you know their sum without knowing the roots?

4. Jul 22, 2007

Gib Z

http://en.wikipedia.org/wiki/Viète's_formulas

Its a bit of grinding but in this case its a degree 4 polynomial so we can get it to the form $(x-\alpha)(x-\beta)(x-\gamma)(x-\delta)$. Where the greek letters are the roots. Now expand that, and equate to co efficients and we can get an expression for the sum of the roots knowing only the coefficients.