# Weird result to a simple derivative question

• Femme_physics
In summary, the conversation discusses finding the value of the derivative at a certain point next to a given function. The function in question is f(x) = x^2 and the derivative is f'(x) = -6. The correct approach is to set f'(x) to -6 and not x. This leads to the point (-3,9) when substituted into the original function.
Femme_physics
Gold Member
I am told to write the value of a the derivative at a certain point next to each function. And to find the point/s

f(x) = x2

f ' (x) = -6

so I figured I just do

f ' (x) = 2x

f ' (-6) = 2 x (-6) = -12

But they tell me the answer is

(-3, 9)

The problem is that f'(x)=-6, and what you plugged in is equivalent to x=-6.

You got the derivative right, but you need to set f'(x) to -6, not x.

Oh... I see! thanks :)

Another way to look at it...
You were given value of the derivative
$f'(x) = -6$ (eq 1)
and you found the derivative of equation $f(x)=x^2$ to be
$f'(x) = 2x$ (eq 2)

The left side of equations 1 & 2 are equal,
therefore the right sides are also,
so -6 = 2x and you can solve for x.

To find y, substitute this x into the original equation of f(x)
and you will obtain the same point you noted from the solution guide.

## 1. What could cause a weird result to a simple derivative question?

There could be several reasons for this. It could be a mistake in the calculation, an error in the initial function, or a misunderstanding of the concept of derivatives.

## 2. How can I check if my calculation of the derivative is correct?

You can double-check your work by using online tools or by solving the problem using a different method. It's also helpful to review the basic rules and properties of derivatives to ensure that you haven't missed anything.

## 3. Is there a way to avoid getting a weird result in a simple derivative question?

Yes, make sure you have a clear understanding of the concepts and steps involved in calculating derivatives. Additionally, double-check your calculations and ask for help or clarification if needed.

## 4. Can a weird result in a derivative question indicate a larger problem with my understanding of calculus?

Not necessarily. It could simply be a mistake in the calculation or a misunderstanding of the problem. However, if you consistently get strange results, it may be helpful to review the fundamentals of calculus and seek additional help from a tutor or teacher.

## 5. What should I do if I am consistently getting weird results in derivative questions?

If you are struggling with derivatives, it's important to seek help from a tutor, teacher, or online resources. It may also be helpful to review the basics of calculus and practice solving different types of derivative problems to improve your understanding and accuracy.

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