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k_rock923
8th June 2004, 01:31
I know that the derivative os cos(4x) is -4sin(4x). . .so does that mean that you just take the four out and put it in front of the derivative of cos?

And i can't figure out for anything how to do 3xsin(3x)

cyu
8th June 2004, 01:43
(3x)(derivative of sin(3x))+(sin3x)(3)

I took calc last year.

k_rock923
8th June 2004, 01:44
that doesn't help because i get stuck on derivative of sin(3x)

edit/ is 3sin(3x) + 9xcos(3x) the final answer?

cyu
8th June 2004, 01:58
Yep. looks right.

griffinn
8th June 2004, 02:01
d (3x sin 3x) / dx
= [d(3x) / dx] * [d (3x sin 3x) / d(3x)]
= 3 * { 3x [d(sin 3x)]/d(3x) + sin 3x * [3x/d(3x)] }
= 3 * ( 3x cos 3x + sin 3x )
= 9x cos 3x + 3 sin 3xSo yeah, it's correct.

ElChevelle
8th June 2004, 02:10
I concur.
:p

k_rock923
8th June 2004, 02:17
something like x(2sinxcosx) becomes 2xsinxcosx right? why is only the 2 multiplied by the x?

griffinn
8th June 2004, 02:19
That's the associative property of multiplication. Basic algebra.

deeder7001
8th June 2004, 02:22
i don't remember how to do advanced math anymore. it's been about a year since i've been in a math class.

k_rock923
8th June 2004, 02:23
thanks guys. once again, basic algebra ruins an otherwise correct problem

cyu
8th June 2004, 02:25
Originally posted by k_rock923
something like x(2sinxcosx) becomes 2xsinxcosx right? why is only the 2 multiplied by the x?
are you talking about the derivative of that?

k_rock923
8th June 2004, 02:30
no. that was part of the answer to xsinsquaredx
so it was x(2sinxcosx)+(sin squared x)1
and that turned into 2xsinxcosx + sin^2 x

if that's wrong, please tell me.

ryan
8th June 2004, 02:56
Ask in the AVS forums, they could most likely help.

griffinn
8th June 2004, 02:59
Sssshhhh... he's got it right, but we're not supposed to tell him!

/me hides

Hollow
8th June 2004, 03:59
k_rock, look up the chain rule in your book. If you don't get it from the book, find someone to show you how it works until you get it. Have random people tell you if you got one question right without understanding the mechanism is wasting your time and ours.

k_rock923
8th June 2004, 19:51
hollow, thats how i solved it. i'm just making sure that i did it right. thanks though.