Find the derivative… - QuestionCove
OpenStudy (anonymous):

Find the derivative of f(x) = -2x^2 + 11x at x = 9

1 year ago
OpenStudy (bibby):

can you find the derivative of -2x^2?

1 year ago
OpenStudy (anonymous):

4 @bibby

1 year ago
OpenStudy (bibby):

so the power rule states that $$\large \dfrac{d}{dx}x^n=nx^{n-1}$$, right? $$\dfrac{d}{dx}-2x^{2}=-2\dfrac{d}{dx}=x^2\\=-2(2x^{2-1})=-2(2x^1)=-2(2x)=-4x$$ do the same thing for the other term

1 year ago
OpenStudy (anonymous):

@bibby that's what I don't get

1 year ago
OpenStudy (bibby):

be more specific, you don't get the power rule?

1 year ago
OpenStudy (anonymous):

how to do it for 11x. and what to do with the 9

1 year ago
OpenStudy (bibby):

when they ask you to find the derivative at x=9, they want you to find $$f'(x)$$ and plug in x=9, so $$f'(x)$$ is the derivative and $$f'(9)$$ is the value at x=9 just rewrite 11x as 11x^1 and then plug it into the power rule

1 year ago
OpenStudy (anonymous):

d/dx + 11x^1 = 11 d/dx = x^1 11(11x ^1-1) = 11(11x) Is this correct so far @bibby

1 year ago
OpenStudy (bibby):

close 1-1=0 so we have $$11x^1\iff 11*1x^{1-1}=11*1*x^0=11*1*1=11$$

1 year ago
OpenStudy (anonymous):

Okay and now I have 4x and 11 so I plug nine in for x then that is my answer? @bibby

1 year ago
OpenStudy (bibby):

f'(x)=-4x+11

1 year ago
OpenStudy (bibby):

yeah

1 year ago
OpenStudy (anonymous):

Thank you so much @bibby

1 year ago