# Math Help - Simple Derivative

1. ## Simple Derivative

I need som help with this.
Im having trouble finding the derivative of (the first image 1.)

The Answer should be (image 2)

But all I get is (image 3)

Were does the √(x-1) come from?

THANKS

2. Hi

$f(x) = (x-4) \: \sqrt{x-1}$

To find the derivative use the formula of the derivative of a product
$(uv)' = u'v + uv'$

Here
$u(x) = x-4$ then $u'(x) = 1$
$v(x) = \sqrt{x-1}$ then $v'(x) = \frac{1}{2\sqrt{x-1}}$

Therefore
$f'(x) = \sqrt{x-1} + \frac{x-4}{2\sqrt{x-1}}$