The curve y=(ax+b)^2(c-x) cuts the x-axis at 3 and touches the x axis at x=9/4. Find the values of a,b,c.

Printable View

- September 14th 2008, 12:50 AMrequalAnother Polynomial question
The curve y=(ax+b)^2(c-x) cuts the x-axis at 3 and touches the x axis at x=9/4. Find the values of a,b,c.

- September 14th 2008, 01:18 AMMoo
Hello,

If one point is on the x-axis, then y=0. So the point (3,0) is a point on the curve.

If it "touches" the x axis at x=9/4, it means that the x axis is tangent to the curve at this point.

This means two things :

- the derivative at this point is 0.

- the point (9/4,0) is a point on a curve.

Now, substitute all this information in the equation. It should give you 3 equations with 3 unknowns. - September 14th 2008, 04:46 AMmr fantastic
- September 14th 2008, 05:12 AMShowcase_22
Could you also take advantage of the fact that where it "touches" the x axis the power of the bracket must be even?

eg.

http://i116.photobucket.com/albums/o...sproblem50.jpg

Since it cuts the x axis the power of the bracket must be odd (in this case 1):

http://i116.photobucket.com/albums/o...sproblem51.jpg

I haven't worked through the rest of it yet but will this way work as well?

(you would need to find another equation linking and c and b or c and a. Differentiation can be used for this).