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:

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

later on he continues, using:

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

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

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 [tex]\frac{1}{2}(a^2+x^2)^{-1/2}(2x)[/tex]

moe darklight

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

and yea those two were typos; I fixed them.