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maths c2 a level watch

1. y=x^2 - k(root)x where k is a constant.
a) Find dy/dx?
I got 2x - k/2 x^-1/2
please check this for me!!!

b) Given that y is decreasing at x=4, find the set of possible values of k?
I know for this that dy/dx <0 but I have no idea what to do!!!!!

ALL HELP IS APPRECIATED!
2. I have looked at this question as it was impossible to do and it is + k not - k. The first part is correct except for the -k. Here is the second part:

f'(4) = 2(4) + k/2*4^-1/2 < 0
= 8 +vk/2*0.25 < 0
= 8 + k/4 < 0
= 32 + k < 0
= k < -32
3. Moved to maths.
4. Can I offer a bit of advice when doing these C2 calculus questions?

Plot the original function and the differential in an on line plotter (fooplot.com is good but there are loads).

As you do questions you will find that you get to see much better what is going on and the ability to 'see functions and their derivatives' will deliver dividends in fewer silly errors as you progress through the core modules.

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