Why are they taking the derivative of 1 - x?

[communitycoll]

Homework Statement

I'm trying to take the derivative of f(x) = 4 / sqrt(1 - x).

Normally if I have trouble I just plug my problem into Wolfram Alpha, and it saves the day.

Homework Equations

However, I don't know why it takes the derivative of 1 - x in the numerator after using the chain rule on 1 / sqrt(1 - x) here:

http://www.wolframalpha.com/input/?i=4+/+sqrt(1-+x)

The Attempt at a Solution

I'll say that I understand everything up until the aforementioned point in the problem.

 

[Mark44]

Homework Statement

I'm trying to take the derivative of f(x) = 4 / sqrt(1 - x).

Normally if I have trouble I just plug my problem into Wolfram Alpha, and it saves the day.

Homework Equations

However, I don't know why it takes the derivative of 1 - x in the numerator after using the chain rule on 1 / sqrt(1 - x) here:

http://www.wolframalpha.com/input/?i=4+/+sqrt(1-+x)

The Attempt at a Solution

I'll say that I understand everything up until the aforementioned point in the problem.

Because they're using the chain rule.
d/dx(1/√u) = d/du(1/√u) * du/dx

 

[communitycoll]

Ah! It took me awhile, but I finally see what they're doing. Thanks.