Question

PLEASE HELP!!!! MEADL WILL BE REWARDED!!!
Find the derivative of f(x) = -4x2 + 11x at x = 10.

Answer choices:
-19.5

-69

30

-300

From my work I was able to calculate 30 but I think I am wrong...

Answer 1

what is your derivative?

Answer 2

find the derivative first and then plug in 10

Answer 3

And is that 4x^2?

Answer 4

Yes @austinL

Answer 5

just use the power rule, this is the easiest derivative of all

Answer 6

Ok, this is a simple derivative.
What did you get for your initial derivative?

Answer 7

I got confused on that part since we are squaring 4x

Answer 8

ok austinL you can have this problem

Answer 9

I'm sorry, if I stepped on your toes I apologize. If you want it, it is all yours!

Answer 10

no no please help CarlyLiberty

Answer 11

Ok, so we want to use the power rule right.
\[f(x) = x^{n}\]
\[f{\prime} (x) = nx^{n}\]

Answer 12

okay so I get that x=10, what is n then in this situation?

Answer 13

ok but you forgot n-1

Answer 14

Oh, right... oopsies
\[f{\prime}(x) = nx^{n-1}\]

Answer 15

In this case \[f(x) = -4x^2 + 11x\]
and so, n for the first half is 2, and the second half is 1.
We have -4x^2, and if we apply the power rule we get -8x, and on 11x, we just get 11.
So, we then have
\[f^{\prime}(x) = -8x + 11\]
From there, we have been given that x=10.

Answer 16

Could you plug that in and see what we get?

Answer 17

yes, we get -80+11=-69
Okay, thank you so much for your help! Like seriously you are a lifesaver!!!!
Its the first steps where I always get confused

Answer 18

No problem! Glad that I could help.