Derivative of inverse tangent function

[biochem850]

Homework Statement

Find derivative of tan^{-1}(\frac{3sinx}{4+5cosx})

Homework Equations

deriviative of tan^{-1}=\frac{U'}{1+U^{2}}

The Attempt at a Solution

I found U'= \frac{12cosx+15}{(4+5cosx)^{2}}

1+U^{2}=1+\frac{9sin^{2}x}{(4+5cosx)^{2}}


I think my components are correct but my answer is still incorrect. Are these basic components even correct (if so I can proceed on my own from here)?

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

 

[tiny-tim]

hi biochem850!

looks ok so far

 

[biochem850]

How do you then simplify:

\frac{U'}{1+U^{2}}?

I've tried to simplify this but with no luck.

 

[biochem850]

Can someone please help me?

 

[tiny-tim]

hi biochem850!

(just got up :zzz:)

well, the (4 + 5cosx)² should cancel and disappear …

show us your full calculations, and then we'll know how to help! ​

 

[biochem850]

\frac{12cosx+15}{(4+5cosx)^{2}}*1+\frac{(4+5cosx)^{2}}{9sin^{2}x}=

\frac{12cosx+15(9sin^{2}x)+(4+5cosx)^{2}(4+5cosx)^{2}}{(4+5cosx)^{2}(9sin^{2}x)}

I'm not sure this is correct.

 

[tiny-tim]

hmm … let's use tex instead of itex, to make it bigger …

\frac{12cosx+15}{(4+5cosx)^{2}} \left(1+\frac{(4+5cosx)^{2}}{9sin^{2}x}\right)=

\frac{12cosx+15}{9sin^{2}x}=\frac{3(4cosx+5)}{9sin^{2}x}

no, that first line is wrong,

the bracket is 1 + 1/U², it should be 1/(1 + U²)

 

[biochem850]

hmm … let's use tex instead of itex, to make it bigger …

no, that first line is wrong,

the bracket is 1 + 1/U², it should be 1/(1 + U²)

I changed my work just before you posted

 

[tiny-tim]

I changed my work just before you posted

but you didn't change the bit i said was wrong

 

[biochem850]

but you didn't change the bit i said was wrong

\frac{12cosx+15}{(4+5cosx)^{2}}\frac{(4+5cosx)^{2}+1}{9sin^{2}x+1}=


\frac{12cosx+15}{9sin^{2}x+1}
I'm not sure this is correct. I really don't understand what you asked me to change.

 

[tiny-tim]

change your original …

1+\frac{9sin^{2}x}{(4+5cosx)^{2}}

… to something with (4 + 5cosx)² on the bottom

 

[biochem850]

change your original …

… to something with (4 + 5cosx)² on the bottom

\frac{(4+5cosx)^{2}+9sin^{2}x}{(4+5cosx)^{2}}?

 

[tiny-tim]

\frac{(4+5cosx)^{2}+9sin^{2}x}{(4+5cosx)^{2}}?

yup!

now turn that upside-down, and multiply, and the (4 + 5cosx)² should cancel

 

[biochem850]

\frac{12cosx+15}{(4+5cosx)^{2}+9sin^{2}x}

In terms of finding the derivative I know I've found it but this can be simplified further.

I don't know if I'm tired or what because I cannot seem to do these very simple problems. This does not bode well for my calculus mark. Would you factor out a 3 in the numerator and then see what will simplify?

 

[tiny-tim]

Would you factor out a 3 in the numerator and then see what will simplify?

no, and i'd be very surprised if this simplifies

(except that you could get rid of the sin² by using cos² + sin² = 1 )

get some sleep! :zzz:​

 

[biochem850]

no, and i'd be very surprised if this simplifies

(except that you could get rid of the sin² by using cos² + sin² = 1 )

get some sleep! :zzz:​

Through some weird algebraic manipulation I simplified down to \frac{3}{5+4cosx}. I believe this is correct. I'm going to sleep.

Thanks so much!