Question

if f(x)=13-8x+root(2x^2) and f'(r)=4 find r?

Answer 1

\[f(x) = 13 - 8x + \sqrt{2x^2}\]
do you know how to find f(r)?

Answer 2

no, that's what is confusing me

Answer 3

hint: change all the x with r

Answer 4

So like this?
\[f(x)=13-8\times4+\sqrt{2\times4 ^{2}}\]

Answer 5

f(r)*

Answer 6

what did you do?

Answer 7

i said replace x with r...not 4

Answer 8

but since f(r)=4 I thought it was that

Answer 9

no...

Answer 10

just replace x with r

Answer 11

\[13−8×r+\sqrt{2×r^{2}}\] ?

Answer 12

right

Answer 13

simplify that

Answer 14

\[13-8r+r \sqrt{2}\]?

Answer 15

yes.

Answer 16

btw...are you sure the question is EXACTLY \[f(x) = 13 - 8x + \sqrt{2x^2}\]

Answer 17

yes, it's exactly that

Answer 18

you're sure...

so \[f(r) = 13 - 8r + r\sqrt 2\]
take the derivative of this

Answer 19

The answer is supposed to be:
\[r=3\sqrt{2}\]
but the derivative gives me:
\[f'(r)=-7\]?