Question

I'm supposed to Find the derivative of f(x) = 12x2 + 8x at x = 9.

Answer 1

I've seen the problem done before, but I'm just confused on the process

Someone said
f(x) = 12x^2 + 8x
f(9) = 12(9) + 8
f(9) = 224

but i don't understand what they did with the extra 'x'

Answer 2

I gather they meant the "slope" at x = 9
in which case, it means, use the "power rule" to get the derivative of that
then in the derivative function, set x = 9

Answer 3

I'm not sure I follow. the derivative function is just (the original one)/x ?

Answer 4

are you doing calc?

Answer 5

Precalc, sir/ma'am

Answer 6

hmmm.... oddly enough.... usually that'd be in calc

well, http://www.math-prof.com/Calculus_1/Calc_Ch_06.asp <--- the power rule is atop

Answer 7

derivative of a constant is 0

Answer 8

Alright, so what I'm seeing is the x^2 (x^n-1) become x^1, and turn x^2 into 2x^1?

Answer 9

\(\bf f(x) = 12x^2 + 8x \implies \cfrac{dy}{dx} = (12 \times 2) x^{2-1} + (8 \times 1) x^{1-1}\\\quad \\
\cfrac{dy}{dx} = 24x+8\\\quad \\
\textit{setting } x =5 \textit{ will give the slope at that point}\)

Answer 10

Wonderful! Thank you!

Answer 11

yw

Answer 12

How might this rule apply with 'x' as a denominator? I suspect it would remain the same since there is no power value?