# Homework Help: Finding the derivative of trig function!

1. Oct 8, 2011

### josh_123

Find dy/dx of these functions:

1. y=sin-1(1/3x)
dy/dx=1/(√1-(1/3x)2 (1/3)
dy/dx=1/(3√1-(1/9x2)

However the book answer is 1/√9-x2). How did they get this?

2. y=sin-1(1/x)
dy/dx=1/√1-(1/x)2 (-1/x2)
If i tried to simplified it by multiply it by x^2/x^2then
dy/dx= -x^2/x4√x2-1 which equal -1/x2√x2-1 However the book answer is -1/absx√x2-1. Can you explain to me how they get this?

3. find dy/dx by implicit differentiation
sin-1(xy)=cos-1(x-y)
so d/dx[sin^-1(xy)=cos^-1(x-y)]
1/√1-(xy)2 (y+x) (dy/dx)=-1/√1-(x-y)2 (0)
(y+x)/√1-(xy)^2 (dy/dx)=0
dy/dx= (- y+x)/(√1-(xy)2)

The formula i used for all these problems are
d/dx[sin-1u]=1/√1-u2 (du/dx)
d/dx[cos-1u]=-1/√1-u2 (du/dx)

~Thank you so much in advance. I really need help before my big test!

2. Oct 8, 2011

### QuarkCharmer

For the first one, they just stuck the 3 in the denominator that came from the chain rule back into the radical.

$$\frac{1}{3\sqrt{1-\frac{1}{9}x^{2}}}$$
$$\frac{1}{\sqrt{9(1-\frac{1}{9}x^{2})}}$$
$$\frac{1}{\sqrt{(9-\frac{9}{9}x^{2})}}$$
$$\frac{1}{\sqrt{(9-x^{2})}}$$

Last edited: Oct 8, 2011
3. Oct 9, 2011

### josh_123

thank you what about the other two?