# Dy/dx if e^2x = sin(x+3y)

by lostintranslation
Tags: derivatives, dy or dx, sinx
 P: 5 I need help with this problem. Find dy/dx if e^2x = sin(x+3y) Any help would be great! I began by taking the natural log of both sides...which may not be correct?? Thanks!
 Sci Advisor HW Helper P: 9,480 then what?
Math
Emeritus
Thanks
PF Gold
P: 39,533
 Quote by lostintranslation I need help with this problem. Find dy/dx if e^2x = sin(x+3y) Any help would be great! I began by taking the natural log of both sides...which may not be correct?? Thanks!
That's certainly one way to do it: you get 2x= ln(sin(x+3y)).
Of course, you will have to use the chain rule on the right side when you differentiate now.
Couldn't you have used the chain rule on both sides of the equation the way it was written? What is $\frac{de^u}{du}$? What is
$\frac{de^u}{du}\frac{du}{dx}$ if u= 2x?

 P: 1,236 Dy/dx if e^2x = sin(x+3y) So $$e^{2x} = \sin(x+3y)$$ When you take the natural log of both sides you get: $$\ln e^{2x} = \ln\sin(x+3y)$$ or $$2x = \ln\sin(x+3y)$$. Now use implicit differentiation to find $$\frac{dy}{dx}$$
 P: 5 Ok, right. So 2x = ln (sin(x+3y)) Then if you take the derivative... 2 = (1/sin(x+3y)) * cos (x+3y) * 3 * dy/dx So dy/dx = 2(sin(x+3y))/ 3(cos(x+3y)) ? Is that right?
 P: 1,236 yes that is correct.
 P: 5 Thanks so much! Does that simplify any farther?
 P: 1,236 I guess you could write it as $$\frac{2\tan(x+3y)}{3}$$
 P: 5 Ok good! That is what I thought. You have been so helpful! Thanks! I have a couple more [shorter] questions, however don't feel obliged to respond if you don't want to... 1. Find dy/dx of y = x^5 * 7^x 2. Find dy/dx of y = (1/2) ^ x 3. Find dy/dx of y = (cosx) ^ x
 P: 1,236 1. Use the chain rule. $$y = x^{5}7^{x}$$. $$y' = x^{5}(7^{x}\ln 7) + 7^{x}(5x^{4})$$ 2. $$(0.5)^{x}$$ is just $$(0.5)^{x}\ln 0.5$$ 3. I'll let you do this one In general, $$\frac{d}{dx} a^{u} = a^{u}\ln a \frac{du}{dx}$$
 P: 5 Right, right. That rule would make those very easy! Thanks bud! I am still having trouble with the cosine one however. I apparently need to use multiple steps to find the derivative.
Math
Emeritus
Thanks
PF Gold
P: 39,533
 Quote by courtrigrad 1. Use the chain rule. $$y = x^{5}7^{x}$$. $$y' = x^{5}(7^{x}\ln 7) + 7^{x}(5x^{4})$$ 2. $$(0.5)^{x}$$ is just $$(0.5)^{x}\ln 0.5$$ 3. I'll let you do this one In general, $$\frac{d}{dx} a^{u} = a^{u}\ln a \frac{du}{dx}$$
Mistype in 2: $$(0.5)^x$$ is just $$e^x\ln 0.5$$.

By the way, I don't see anything gained by taking the logarithm in the first problem you posted. It is just as easy to differentiate both sides of
e2x= sin(x+ 3y) the way it stands:
2e2x= cos(x+ 3y)(1+ y') so
$$1+ y'= \frac{2e^{2x}}{cos(x+3y}}$$
and
$$y'= \frac{2e^{2x}}{cos(x+3y}}- 1$$
HW Helper
P: 1,422
 Quote by lostintranslation 2 = (1/sin(x+3y)) * cos (x+3y) * 3 * dy/dx So dy/dx = 2(sin(x+3y))/ 3(cos(x+3y)) ? Is that right?
Nope, that's {B]not[/B] correct!
$$2 = \frac{\cos (x + 3y)}{\sin (x + 3y)} \left( 1 + 3 \frac{dy}{dx} \right)$$
You forgot the (1 + 3 dy / dx)
 Quote by courtrigrad yes that is correct.
???
How on Earth can that be called correct?
 Quote by HallsofIvy Mistype in 2: $$(0.5)^x$$ is just $$e^x\ln 0.5$$.
That's also a mistype.
0.5x = ex ln(0.5)
 Quote by HallsofIvy e2x= sin(x+ 3y) the way it stands: 2e2x= cos(x+ 3y)(1+ y')
By the way, there's a typo there:
e2x= sin(x+ 3y)
Differentiate both sides with respect to x gives:
2e2x= cos(x+ 3y)(1+ 3y')

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To find dy / dx of y = cosxx. Just do the same:
cosxx = ex ln(cos(x))
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Or, you can do it a bit differently:
y = cosxx
Take the logarithm of both sides gives:
ln y = x ln(cos x)
Then you can differentiate both sides with respect to x, and see what you get.
 P: 1,236 sorry for the typos Thanks HallsofIvy and VietDao for correcting me.

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