Finding a point on a slope that meets a function - tricky one though

[Femme_physics]

Homework Statement

Find the coordinates of point A and B
The function is

f(x) = x² -6x +a

(where "a" is a parameter)

The equation tangent line to the function at point A is
y = -2x +1

The equation tangent line to the function at point B
y = 2x -11

http://img864.imageshack.us/i/calc01.jpg/

The Attempt at a Solution

In order to find A I just need to set the function equal to the line equation at A, since this is the point where they meet. The problem is that "a" is a parameter. Until I know a I can't solve this equation. I can see "a" as an unknown, but then I got 1 equation and 2 unknowns.

I'm rather confused as to how to find this point without "a" being defined to me.

Any help?

 

[I like Serena]

What's the derivative of f(x)?

To find the x coordinate of A you need f'(x) = -2.
Can you solve that?

 

[Femme_physics]

How will the derivative of f(x) help me find that point? Do I set the derivative of the sad-smile parabola's function equal to the slope function?

To find the x coordinate of A you need f'(x) = -2.

Hmm, wait, the derivative of the slope function gives you the point A?

 

[Amok]

What's the derivative of f(x)?

To find the x coordinate of A you need f'(x) = -2.
Can you solve that?

Isn't this precalculus math? Have you taken calculus Femme Physique?

How will the derivative of f(x) help me find that point? Do I set the derivative of the sad-smile parabola's function equal to the slope function?
Hmm, wait, the derivative of the slope function gives you the point A?

The derivative of your function at a certain point gives you the slope of the tangent at that point. The slope of your first tangent line is -2, so if you set f'(x) = -2 and solve for x, you should find the abscissa of point A.

 

[I like Serena]

How will the derivative of f(x) help me find that point? Do I set the derivative of the sad-smile parabola's function equal to the slope function?



Hmm, wait, the derivative of the slope function gives you the point A?

The derivative of a function is the slope of that function.

In point A the slope of f(x) must be equal to the slope of the given tangent line in A.
This gives you 1 equation with 1 unknown...

 

[Femme_physics]

Isn't this precalculus math? Have you taken calculus Femme Physique?

"Femme_Physique"? *chuckles* :D

I know calc, I just thought it's a problem where I don't have to use calculus.

The derivative of your function at a certain point gives you the slope of the tangent at that point. The slope of your first tangent line is -2, so if you set f'(x) = -2 and solve for x, you should find the abscissa of point A.

Oh, I see, it makes sense actually, thanks! I'll try it right now :)

And thanks to you ILS as always!

 

[Mentallic]

You don't need calculus for this problem. Think about this, if you try to equate a line and a parabola (to see where they cut), you'll end up solving a quadratic equation. There could either be 2 solutions, 1 solution, or none at all. What do each of these mean in terms of the parabola and line?

 

[Femme_physics]

You don't need calculus for this problem. Think about this, if you try to equate a line and a parabola (to see where they cut), you'll end up solving a quadratic equation.

Yes-- but I don't have "a", "a" is a parameter.

I was able to solve it only using calculus.

 

[Mentallic]

Yes I know that, you need to find a first before you can find the coordinates of intersection A and B. Just try to equate one of the lines with the parabola, and take into consideration when finding the roots (which will be in terms of a) that the line is tangent to it.

 

[Femme_physics]

Makes sense. "a" is how much the function moves to the right, right? And I can also find a but finding the minima point, correct?

Hmm..wait, I turn up with a = 3, but the answer book says 5. I did do what you said

-2x+1 = 2x -11
4x = 12
x = 3

 

[Mentallic]

No, a is how much it moves up and down. If you have some parabola y_1=ax^2+bx+c then the parabola y_2=ax^2+bx+c+1=y_1+1 will move everything up 1 more unit.

You found where the two lines intersect each other by equating them together, instead equate the parabola and one of the lines.

 

[Femme_physics]

But if I equate the parabola's function and one of the slope's function, I still have 1 equation and 2 unknowns (x and a are unknown)

 

[Mentallic]

But if I equate the parabola's function and one of the slope's function, I still have 1 equation and 2 unknowns (x and a are unknown)

I've already explained this. You'll be solving for x (the answer will be in terms of a) and then you need to take one more detail into consideration:

You don't need calculus for this problem. Think about this, if you try to equate a line and a parabola (to see where they cut), you'll end up solving a quadratic equation. There could either be 2 solutions, 1 solution, or none at all. What do each of these mean in terms of the parabola and line?

 

[I like Serena]

The equation you would have is:

x² - 6x + a = -2x + 1

or:
x² - 4x + (a - 1) = 0

This is a quadratic equation in x (which includes an "a", but we'll ignore that for now).

The solution is (see also http://en.wikipedia.org/wiki/Quadratic_equation):

x=\frac {-(-4) \pm \sqrt {(-4)^2 - 4 \cdot 1 \cdot (a-1)} } {2 \cdot 1}

This yields 2 solutions, but only if the sqrt-part is greater than zero.
In particular if the sqrt-part equals zero, there is exactly 1 solution, which is what we're looking for.

So we have:
(-4)^2 - 4 \cdot 1 \cdot (a-1)} = 0

From this equation you can find a, and then you can also find x.

 

[Femme_physics]

I kinda get stuck here, unsure how to take a square root of a number with an unknown

 

[I like Serena]

I kinda get stuck here, unsure how to take a square root of a number with an unknown

That's right, you cannot take the square root of a number with an unknown.

The "trick" is that you know something else: there must be only 1 solution.

Try it for instance with a = 0 and also with a = 2.
Which (and in particular: how many) solutions do you get?

[edit]btw, I just saw that your calculation that came out to (12a - 12) is not quite right.
I guess we can get to that in a minute.[/edit]

 

[Femme_physics]

I plug (-4)²-4 * 1 and I get 12

So from there it's 12(a-1) = 12a -12

That's for the numerator under the square root

 

[I like Serena]

I plug (-4)²-4 * 1 and I get 12

So from there it's 12(a-1) = 12a -12

That's for the numerator under the square root

There's a rule that multiplying comes before adding/subtracting.

So (-4)² - 4 * 1 * (a - 1) = (-4)² - (4 * 1 * (a - 1))
(Notice that I've added a couple of parentheses to emphasize the order of evaluation.)

This comes out to:
16 - (4 * (a - 1))

Since you have an unknown, you can't calculate e.g. (a - 1) yet, but what you can do is rewrite the expression, eliminating the parentheses.

That is:
16 - (4 * a - 4 * 1) = 16 - (4a - 4)

[edit]That is because:
2 * (3 + 4) = (2 * 3) + (2 * 4)
Try it if you don't believe me! :)
[/edit]

 

[Mentallic]

Femme_physics you should revise your algebra!

 

[Femme_physics]

*smacks forehead* It's the calculator's fault! I mean, the way I tried to solve it using a calculator instead of writing everything down.

I'll try it again then. Thanks :)

 

[Femme_physics]

So, there it is. I'm still getting stuck with the same concept that I can't take the square root of an unknown.

That's right, you cannot take the square root of a number with an unknown.

The "trick" is that you know something else: there must be only 1 solution.

Try it for instance with a = 0 and also with a = 2.
Which (and in particular: how many) solutions do you get?

Hm. But I can't just set "a" to whatever I want, right?

 

[I like Serena]

So, there it is. I'm still getting stuck with the same concept that I can't take the square root of an unknown.

I'm going to take this one step at a time.

You write:
16 - 4(a - 1)

and then you make from this:
16 - 4a - 1

This is not right.
If you for instance fill in "a = 2", you'll get:
16 - 4(2 - 1) = 16 - 4(1) = 12
respectively:
16 - 4 x 2 - 1 = 16 - 8 - 1 = 7

You need to find something that if you fill in a value for "a" the results before and after match.

Hm. But I can't just set "a" to whatever I want, right?

Can you set "a" to whatever you want?
Well, yes and no.

Yes, you can set "a" to any value you want, and see what comes out.
But ultimately "a" must have a value such that the parabola will "touch" the lines in points A and B.

 

[Femme_physics]

Okay, so I was rush at getting to the next step and didn't solve correctly for
16 - 4(a - 1)

Clearly it's 16 -4a +4
Which then becomes
20 -4a

You need to find something that if you fill in a value for "a" the results before and after match.

So this is a guessing game? I just start from a=1 till a=1000 until I find something that matches?

 

[I like Serena]

Okay, so I was rush at getting to the next step and didn't solve correctly for
16 - 4(a - 1)

Clearly it's 16 -4a +4
Which then becomes
20 -4a

So this is a guessing game? I just start from a=1 till a=1000 until I find something that matches?

All right!
Let's make it a guessing game! :)
Take a guess for "a" between 1 and 1000.
Can you give me a number?

[edit]I pick a = 1. What's yours?[/edit]

[edit2]Can you give me the solutions for x for your choice?[/edit2]

 

[Femme_physics]

Okay, I gave it a shot, and after three random guesses it appears that a=5, where the whole shebang under the square root goes to zero, that gives the same 2 results.

So I'm always supposed to get the whole shebang under the square root to get to 0 to find a?

 

[I like Serena]

Okay, I gave it a shot, and after three random guesses it appears that a=5, where the whole shebang under the square root goes to zero, that gives the same 2 results.

So I'm always supposed to get the whole shebang under the square root to get to 0 to find a?

Only if you want the equation to have exactly 1 solution, which now we do.

It's a kind of special case in which you can solve 2 unknowns from 1 equation, but only with the extra information that the parabola "touches" the line.

 

[Femme_physics]

It's a kind of special case in which you can solve 2 unknowns from 1 equation, but only with the extra information that the parabola "touches" the line.

Woahhhhh...I'm on math drug...

Many, many thanks for getting me there ILS!

 

[I like Serena]

Earlier, you wrote this.

Yes-- but I don't have "a", "a" is a parameter.

I was able to solve it only using calculus.

Did you get the same values for x and a (using calculus)?

 

[Femme_physics]

Well, I just needed to find a, not x! I'm not sure what you mean

By the way, now it's really getting into calculus, for the last question I need to find the area that's caged above the slopes but below the function (graphed in the original upload).

I persume I take the integral of the parabola's function from 2 to 4, then reduce it by the integrals of the two slopes from 2 to 4. Does it make sense?

 

[I like Serena]

Well, I just needed to find a, not x! I'm not sure what you mean

The problem statement asks for the coordinates of A and B.
You have just not only found a = 5, but also x = 2 for point A.
You're not entirely finished yet, because you also need the y-coordinate of A, and the coordinates of B.


However, my original response at the start of the thread was:

To find the x coordinate of A you need f'(x) = -2.

In point A the slope of f(x) must be equal to the slope of the given tangent line in A.
This gives you 1 equation with 1 unknown...

This is first step in the "calculus" way of solving the problem using the derivative, which is actually easier.
It's before you were set on the course to intersect a parabola with a line, triggered by the comment of Mentalic.

Since you're learning calculus this seems to be a good place to practice it too. :)

By the way, now it's really getting into calculus, for the last question I need to find the area that's caged above the slopes but below the function (graphed in the original upload).

I persume I take the integral of the parabola's function from 2 to 4, then reduce it by the integrals of the two slopes from 2 to 4. Does it make sense?

Yes, that makes perfect sense!

Take note however that you need to take the integral of the first slope from 2 to 3, and the integral of the second slope from 3 to 4.