# Is this correct

1. Oct 4, 2008

### afcwestwarrior

y=-1/2 x cosx

y'=1/2 x sinx + cos x -1/2

y"= 1/2 x cosx - sin x 1/2 + cos x + -1/2 - sin x

y"= 1/2 x cos^2x

I used the product rule (fg)'=fg'+gf'

2. Oct 4, 2008

### Unassuming

I am an amateur myself but I recall (hopefully) that cos(x) differentiates to -sin(x)

EDIT: I didn't see the negative sorry

3. Oct 4, 2008

$$y' = \frac 1 2 x \sin x - \frac 1 2 \cos x$$

that is correct.

I do not believe your second derivative is correct.

4. Oct 4, 2008

### afcwestwarrior

Will the second derivative be
y"=1/2 x cos x + sin x * 1/2 - 1/2 - sin x

y"=1/2 x cos x + 1/2 sin x + 1/2 sin x

5. Oct 4, 2008

### afcwestwarrior

Well I'm doing differential equations and I have to match it with Y" + y= sin x

6. Oct 4, 2008

### Unassuming

y''= (1/2)xcosx + sinx

I didn't get the ^2x part.

7. Oct 4, 2008

### afcwestwarrior

How'd you get that second derivative. I did it wrong how'd u do it, you did it right

8. Oct 4, 2008