# Math Help - common tangent line?

1. ## common tangent line?

find the two points on the curve
y=x4-2x2​-x that have a common tangent line

2. ## Re: common tangent line?

Originally Posted by pnfuller
find the two points on the curve
y=x4-2x2​-x that have a common tangent line
Try the two points:

x1=-1, x2=1

3. ## Re: common tangent line?

what do i do with those two points?
Originally Posted by MaxJasper
Try the two points:

x1=-1, x2=1

4. ## Re: common tangent line?

The equation of the line to a function at $(x_n,f(x_n)$ is given in slope intercept form as:

$y=f'(x_n)x+f(x_n)-x_nf(x_n)$

Two lines are the same if they have the same slope and y-intercept. Using the points $(x_1,f(x_1))$ and $(x_2,f(x_2))$ will give you two equations and two unknowns, from which you can find two distinct points.

5. ## Re: common tangent line?

Originally Posted by pnfuller
find the two points on the curve
y=x4-2x2​-x that have a common tangent line
The slope of the tangent line is:
$m = \frac{dy}{dx} = 4x^3-4x-1$

$f'(a) = 4 a^3 -4a -1$

$f'(b) = 4 b^3 -4b -1$

$f(a) = a^4 -2a^2 -a$

$f(b) = b^4 -2 b^2 -b$

Now:

$f'(a) =\frac{f(b) - f(a)}{b -a}$

And:
$f'(b) =\frac{f(b) - f(a)}{b -a}$

So now the equation becomes:

$(4 a^3 -4a -1)(b - a) = (b^4 -2 b^2 -b) - (a^4 -2a^2 -a)..................(1)$

$(4 b^3 -4b -1)(b - a) = (b^4 -2 b^2 -b) - (a^4 -2a^2 -a)..................(2)$

Solving these two equations by maple we find $a = -1 \text{ and } b = 1$ or $a = 1 \text{ and } b = -1$

6. ## Re: common tangent line?

what do you mean solve it with maple?
Originally Posted by x3bnm
The slope of the tangent line is:
$m = \frac{dy}{dx} = 4x^3-4x-1$

$f'(a) = 4 a^3 -4a -1$

$f'(b) = 4 b^3 -4b -1$

$f(a) = a^4 -2a^2 -a$

$f(b) = b^4 -2 b^2 -b$

Now:

$f'(a) =\frac{f(b) - f(a)}{b -a}$

And:
$f'(b) =\frac{f(b) - f(a)}{b -a}$

So now the equation becomes:

$(4 a^3 -4a -1)(b - a) = (b^4 -2 b^2 -b) - (a^4 -2a^2 -a)..................(1)$

$(4 b^3 -4b -1)(b - a) = (b^4 -2 b^2 -b) - (a^4 -2a^2 -a)..................(2)$

Solving these two equations by maple we find $a = -1 \text{ and } b = 1$ or $a = 1 \text{ and } b = -1$

7. ## Re: common tangent line?

Maple is a program that does mathematical calculations. Here's a link:

Maple 16 by Maplesoft

- Hollywood