Attack of my horrid math skills, pt. 1

[moe darklight]

I thought I should start numbering these... anyway, here's another brain-fart where I miss something obvious:

in the book the dude uses:

$$\[<br /> f(x) = \frac{1}{{30}}\sqrt {a^2 + x^2 } + \frac{1}{{60}}(b - x)<br /> \]$$

later on he continues, using:

$$\[<br /> f'(x) = \frac{1}{{30}}\frac{1}{2}(a^2 + x^2 )^{ - 1/2} (2x) - \frac{1}{{60}}<br /> \]$$

where'd that 2x come from? isn't the derivative of $$\[<br /> \sqrt {a^2 + x^2 } <br /> \]$$ just $$\[<br /> \frac{1}{2}(a^2 + x^2 )^{ - 1/2} <br /> \]$$?

thanks

 

[d_leet]

Well assuming that you mean x instead of b, that 2x comes from the chain rule.

 

[bob1182006]

yea chain rule but also he seems to have confused x for b in the derivative, should be $$\frac{1}{2}(a^2+x^2)^{-1/2}(2x)$$

 

[moe darklight]

blah, I'm an idiot . o well, haha thanks.

and yea those two were typos; I fixed them.