# Math Help - bit of help needed on cos-1 differentiation

1. ## bit of help needed on cos-1 differentiation

find dy/dx when x=2 if
y=cos-1 (x)
i understand that
-1 du
squrt 1-u^2 dx
but now i will get a negative number at the bottom and i cant get a square root of a negative number thank you 4 your time

2. $x = 2$ is not in the domain of the inverse trig function $y = \cos^{-1}{x}$.
the domain is $-1 \leq x \leq 1$

you sure it isn't ...

$y = (\cos{x})^{-1} = \frac{1}{\cos{x}} = \sec{x}$ ?