Derivation of trigonometric functions

f(x) = sqrt(x) * cos2x
f'(x) = making me insane

any ideas? I should know this

Just a poor boy

[acropolis]

Originally posted by Just a poor boy
f(x) = sqrt(x) * cos2x
f'(x) = making me insane

any ideas? I should know this

Just a poor boy

1) differentiation (unless there's a terminology difference...)

2) (1/2)(X^(-1/2))*cos (2x) + (x^(1/2)) * 2 * -sin(2x)

i think. but it's just the basic rule

h(x) = f(x) * g(x)
h'(x) = f'(x) *g(x) + f(x) * g'(x)

thats what Im doing tho, and Im still just a poor boy =/

[Ragnarak]

will try to explain what acropolic said in words:

treat it as 2 separate bits

bit a is the root x bit

bit b is the trig term

first you differentiate bit a, and multiply it by bit b

then you differentiate bit b and multiply it by bit a

then add it all together

this gives:

0.5X^(-0.5)*cosx as the first part

and

-2X^(0.5)*sinx as the second part

giving as your final answer:

0.5X^(-0.5)*cosx - 2X^(0.5)*sinx

if you're still having problems grab a maths text book and look up 'chain rule' and work through the examples

[acropolis]

Originally posted by Ragnarak

if you're still having problems grab a maths text book and look up 'chain rule' and work through the examples

tis not the chain rule tho.

i was gonna put that too, but it aint so i didnt.

its like the 'multiplication rule' or something gay like that.

[Ragnarak]

Originally posted by acropolis
tis not the chain rule tho.

i was gonna put that too, but it aint so i didnt.

its like the 'multiplication rule' or something gay like that.

hmm

maybe

i'll take your word for it

thing is Im getting the exact same answer as you guys are, yet it doesnt fit on mistar graphical calculator

[Sarina_Joy]

Graphics calculators are a stupid pointless waste of money.

[Ragnarak]

Originally posted by Just a poor boy
thing is Im getting the exact same answer as you guys are, yet it doesnt fit on mistar graphical calculator

heh

what are you plotting on the calc? the final answer?

just zoom in/out till it works

(or use Maple or MatLab or something to do it on your computer )

and also the only way for me to check if my derivations are A-OK

[acropolis]

Originally posted by Just a poor boy
thing is Im getting the exact same answer as you guys are, yet it doesnt fit on mistar graphical calculator

it doesn't "fit"???

what does that mean?

ps i found the rule to be called "the rule for differentiating a product" which is even gayer than gay.

[Christian]

Originally posted by Sarina_Joy
Graphics calculators are a stupid pointless waste of money.

Never.

Originally posted by Ragnarak
heh

what are you plotting on the calc? the final answer?

just zoom in/out till it works

(or use Maple or MatLab or something to do it on your computer )

Im plotting f(x) and f'(x)
with the answer we all seem to be getting, f(x)=EXTREME is not at the same x value as f'(x)=0

or so my TI wants me to believe

also I must leave but I shall be back

[Bunga]

f(x) = u.v
where u = sqrt(x)
v = cos(2x)

f'(x) = f'(u).v + f'(v).u
f'(x) = [0.5x^(-0.5)][cos(2x)] + [-2sin(2x)][sqrt(x)]

Simple. But I hate maths on a comp. When I do this on the graphic calculator it works fine. The two lines check.

[acropolis]

Originally posted by Just a poor boy
Im plotting f(x) and f'(x)
with the answer we all seem to be getting, f(x)=EXTREME is not at the same x value as f'(x)=0

or so my TI wants me to believe

what the hell is he possibly talking about?

(ps the graphikals calcs rock)

[Supernova9]

Sounds like the Product rule, from P3 maths if I'm not mistaken,

why you're plotting F(x) against F`(x) I'm not sure though, but if it is the product rule, Bunga is right.

[Marilyn Manson]

MATHS IS NOT FOR NORMAL PEOPLE.

[LHC]

Originally posted by Marilyn Manson
MATHS IS NOT FOR NORMAL PEOPLE.

Neither is makeup on males.