# Thread: Derivatives of inverse trig function

1. ## Derivatives of inverse trig function

I need help taking the derivative of y = arctan(x+1)
This is what I do
y = arctan(x+1)
x=tan(y+1)
cos^2(y+1) = y'
cos^2(arctan(x+1)+1) = y'
cos^2(((1-sec^2x)^0.5)+2))=y'

I am suppose to get a polynomial but I cannot take the derivative of it to get one. I just get stuck at the last step. Any help?

2. ## Re: Derivatives of inverse trig function

Originally Posted by Barthayn
I need help taking the derivative of y = arctan(x+1)
This is what I do
y = arctan(x+1)
x=tan(y+1)
cos^2(y+1) = y'
cos^2(arctan(x+1)+1) = y'
cos^2(((1-sec^2x)^0.5)+2))=y'

I am suppose to get a polynomial but I cannot take the derivative of it to get one. I just get stuck at the last step. Any help?
The tan and arctan functions don't work that way. You can only take the tangent of the whole expression

$\displaystyle \tan(y) = \tan(\arctan(x+1))$

$\displaystyle \tan(y) = x+1$

$\displaystyle x = \tan(y) - 1$

When you differentiate wrt x you should get: $\displaystyle 1 = \sec^2(y)y'$