Calc Tangent Problem: Find Lines Tangent to f(x) at x=-1

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In summary, the conversation was about finding the equations of the tangent lines to the curve f(x)=a(7-x^2) at x=-1. The solution involved taking the derivative, which was found to be -2ax, and then using the point-slope form to get the equation y=2ax+8a. However, there was a mistake in calculating the y-intercept, which should have been 6a instead of 8a.
  • #1
coookiemonste
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Homework Statement


Let f(x)=a(7-x^2)
find in terms of a, the equations of the lines tangent to these curves at x=-1.

Homework Equations


?

The Attempt at a Solution


So I took the derivative of f(x).
f'(x)=a(-2x)+7-x^2
then i plugged in -1 into f'(x)
and i got f'(-1)=2ax+7+1=2ax+8
but the answer to the problem is y=2ax+8a.
I don't know where I went wrong.
 
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  • #2
It went wrong at the part where you took the derivative. Write out the brackets and then ask yourself what's [tex]\frac{d7a}{dx}, \frac{-dx^2}{dx}[/tex].
 
  • #3
The derivative of a(7 - x²) is -2ax.

The slope of a line is not enough to determine the equation of the line. If you have the slope, and also its height at a point b, call it c, then the equation of the line will be:

y = c + (x - b)(slope)

Do you recognize this?
 
  • #4
ok
so f'(x)=-2ax which is the slope.
then to get the set of points
you know x=-1,
so f(-1)=8a.
but if you used point slope format
wouldnt you get y-8a=-2ax(x+1) ?
 
  • #5
Yes, except you have to replace -2ax in your equation with -2a(-1) since you want the slope at -1.
 
  • #6
o ok.
but wouldn't you still get y-8a=2a(x+1)
which is y=2ax+10a?
 
  • #7
You calculated f(-1) wrong. It should be 6a not 8a.
 
  • #8
thanks!
 
  • #9
No problem. :smile:
 

FAQ: Calc Tangent Problem: Find Lines Tangent to f(x) at x=-1

What is a tangent line?

A tangent line is a line that touches a curve at a single point, without crossing through it. It represents the instantaneous rate of change, or slope, of the curve at that point.

Why is finding tangent lines important?

Tangent lines are important because they can help us understand the behavior of a curve at a specific point. They also allow us to find the slope of a curve at a certain point, which is useful in many applications, such as calculating rates of change and optimizing functions.

How do you find the slope of a tangent line?

The slope of a tangent line can be found by taking the derivative of the function at the given point. In this case, we would use the derivative of f(x) to find the slope of the tangent line at x=-1.

What is the process for finding lines tangent to f(x) at x=-1?

To find lines tangent to f(x) at x=-1, we first need to take the derivative of f(x) and evaluate it at x=-1 to find the slope of the tangent line. Then, we can use the point-slope formula (y-y1 = m(x-x1)) to find the equation of the tangent line passing through the point (-1, f(-1)).

Can there be more than one line tangent to a curve at a given point?

Yes, there can be more than one line tangent to a curve at a given point. This can happen when the curve has a sharp point, such as a cusp or corner, at that point. In this case, there may be multiple lines that touch the curve at that point without crossing through it.

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