Question

Find the derivative of f(x) = -12x2 + 9x at x = 6

Answer 1

Sure, lets find the derivative first then we can substitute the x = 6 afterwards. So what's the derivative, any idea?

Answer 2

um no ; ;

Answer 3

Hint: Use power rule

Answer 4

i am a bit confused with this module ^.^;

Answer 5

Well power rule suggests \[\frac{ d }{ dx } x^n \implies n x^{n-1}\]

Answer 6

how i started of this equation was with the: f(x)= (x+h) equation

Answer 7

was i wrong 0.o

Answer 8

Oooh do you have to use the definition of a derivative, using limits?

Answer 9

i got f(x)= (-12(x+h)^2+9(x+h)- (12x^2+9x))/h

Answer 10

um the directions dont specify :/

Answer 11

well we can just try it whatever way you think is more efficient :)

Answer 12

Just to make sure what you're doing, you're using \[f'(a) = \lim_{h \rightarrow 0} \frac{ f(a+h)-f(a) }{ h }\]

Answer 13

the answer choices are:
-112.5
-135
-90
-108

Answer 14

Lol I guess you have to do it this way as it's the definition of a derivative, that's what your teacher expects, but once you learn the rules you'll be like wow -.- I spent so much time to do something that can be done in seconds.

Answer 15

I want to be able to do it in seconds xD

Answer 16

Do you know of the power rule?

Answer 17

no :(

Answer 18

I think you should learn the definition of derivative, that is what your test will expect.

Answer 19

well the test doesnt ask to show are work, as long as we get the answer it shouldnt matter how we did it :P

Answer 20

i tend to make mistakes in long problems >.<

Answer 21

Whats the point of the answer if you don't understand the process, the answer is not as important as understanding how it works, but anyways up to you :) how would you like to proceed this?

Answer 22

Hmm, if you teach me the "power rule" will i be able to answer other problems like this?

Answer 23

i just want to be able to answer these types of questions xD

Answer 24

There's many rules, that's just one, you can look them up

Answer 25

\[\frac{ d }{ dx } x^n \implies n x^{n-1} \] power rule here

Answer 26

Would you mind just going over both ways?

Answer 27

like with the power rule and how the test expaects

Answer 28

Yeah that would be best :)

Answer 29

xD <3

Answer 30

I'll be back in 2 min, just eating :)

Answer 31

np

Answer 32

this a homework not a test :P

Answer 33

Ok back, thanks for waiting

Answer 34

no problem :)

Answer 35

So we have to use the definition of a limit right? So can you show me what that is please :)

Answer 36

If possible try to draw it out rather than type as it's too messy then

Answer 37

um what if i dont know what that is ; ;

Answer 38

i am not good with names ; ;

Answer 39

It was what you were essentially doing

Answer 40

Just show me w/e you were doing

Answer 41

like the distance between to points?

Answer 42

i am not really sure what you are asking for :/

Answer 43

I'm asking you to show me what you were doing for this problem already lol

Answer 44

ohhh wait i think i know what that is

Answer 45

isnt it the value a ffunction approaches?

Answer 46

for example

Answer 47

go