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# Maths C1 - Rates of Change question! watch

1. A function f is defined by f(x)=4-3x-x^3

a) Find the derivative f'(x)

Which I found to be f'(x)=-3x^2-3 After rearranging.

b) Use your answer from (a) to deduce whether the function is an increasing or decreasing function.

Do I make x<o and go from there or is this wrong?

Thanks for the help.
2. doesn't it give you a value of x?
3. (Original post by Jfranny)
doesn't it give you a value of x?
This as what I though..

4. f'(x) is always going to be negative, so the gradient of f(x) is always negative - so it is a decreasing function
5. (Original post by markioe)
For all values of x>1 its a positive gradient (it should say x>1 somewhere in the question) thus the function is increasing as x increases.
Are you reading it right? The derivative is -3x^2 - 3, so it's decreasing at all points.
6. (Original post by starofale)

f'(x) is always going to be negative, so the gradient of f(x) is always negative - so it is a decreasing function
Thanks I'll be sure to rep you!
7. (Original post by starofale)

f'(x) is always going to be negative, so the gradient of f(x) is always negative - so it is a decreasing function
Could you point me in the right way for this question?

Find the set of values of x for which the function h, where h(x)=2-(x+3)^2 is increasing.
8. Oh misread, indeed it is decreasing.
9. In this case f'(x) can only be negative...any x value will do.
10. (Original post by econ1)
Could you point me in the right way for this question?

Find the set of values of x for which the function h, where h(x)=2-(x+3)^2 is increasing.

h(x) will be decreasing when h'(x) is negative. Find h'(x) and solve for h'(x)<0

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