# Thread: Horizontal line

1. ## Horizontal line

Let f(x)=ax^3+bx^2+cx+d , find conditions for a,b.c and d so that

a) f has no horizontal tangent line.
b) f has aunique horizontal tangent line.
c) f has tow horizontal tangent lines

2. Originally Posted by BAHADEEN
Let f(x)=ax^3+bx^2+cx+d , find conditions for a,b.c and d so that

a) f has no horizontal tangent line.
b) f has aunique horizontal tangent line.
c) f has tow horizontal tangent lines
If you take the derivative you get

$f'(x)=3ax^2+2bx+c$

So a graph has a horizontal tangent when the derivative is equal to zero.

so we need to analyze the discriminant of the quadratic.

Can you finish from here?

3. Originally Posted by TheEmptySet
If you take the derivative you get

$f'(x)=3ax^2+2bx+c$

So a graph has a horizontal tangent when the derivative is equal to zero.

so we need to analyze the discriminant of the quadratic.

Can you finish from here?
How could I find the three conditions? thank you for help

4. Originally Posted by BAHADEEN
How could I find the three conditions? thank you for help
The discrimiant is

$(2b)^2-4(3a)(c)=4b^2-12ac=4(b^2-3ac)$

This will have no solutions when the discrimiant is negative.

This will have exactly one solution when the discrimiant is equal to zero.

This will have two solutions when the discrimiant is positive.