Algebra Help for a Derivative

• October 6th 2013, 06:29 PM
IdentityProblem
Algebra Help for a Derivative
There's probably easier ways to solve this derivative by using logarithmic rules, but using the method that I took, do you see where I go wrong with the algebra?

It would greatly help to see what errors I making! I think the same errors pop up when doing problems like this. Thank you!
• October 6th 2013, 06:46 PM
SlipEternal
Re: Algebra Help for a Derivative
I can't really read what you wrote. It looks like the problem is $y = \ln\left( \dfrac{(x^2+1)^5}{\sqrt{2-x}} \right)$. Then, when you get to line 6, it appears that $\dfrac{1}{\left(\dfrac{(x^2+1)^5}{\sqrt{2-x}}\right) }$ becomes $\dfrac{1}{\sqrt{2-x} (x^2+1)^5}\right) }$. That is wrong. It should become $\dfrac{\sqrt{2-x}}{(x^2+1)^5}$
• October 6th 2013, 06:51 PM
IdentityProblem
Re: Algebra Help for a Derivative
Yes! That's what puzzles me a lot. I don't have that part down in my head yet.

When dividing a complex fraction like that, I confuse the order of what group divides what.

Any advice on that? I took it that 1 divides (x^2 +1)^5 first.
• October 6th 2013, 07:05 PM
SlipEternal
Re: Algebra Help for a Derivative
Multiply numerator and denominator by $\sqrt{2-x}$. You get $1\cdot \sqrt{2-x}$ in the numerator and $\dfrac{(x^2+1)^5}{\sqrt{2-x}} \sqrt{2-x}$ in the denominator. The $\sqrt{2-x}$ and the $\dfrac{1}{\sqrt{2-x}}$ cancel each other out.
• October 6th 2013, 08:33 PM
ibdutt
Re: Algebra Help for a Derivative
• October 6th 2013, 09:02 PM
SlipEternal
Re: Algebra Help for a Derivative
Looks good with a quick glance
• October 6th 2013, 10:24 PM
Prove It
Re: Algebra Help for a Derivative
Quote:

Originally Posted by IdentityProblem
There's probably easier ways to solve this derivative by using logarithmic rules, but using the method that I took, do you see where I go wrong with the algebra?

It would greatly help to see what errors I making! I think the same errors pop up when doing problems like this. Thank you!

Why don't you use the logarithm laws to make life easier on yourself?

\displaystyle \begin{align*} \ln{ \left[ \frac{ \left( x^2 + 1 \right) ^5 }{\sqrt{1 - x} } \right] } &= \ln{ \left[ \left( x^2 + 1 \right) ^5 \right] } - \ln{ \left( \sqrt{1 - x} \right) } \\ &= 5\ln{ \left( x^2 + 1 \right) } - \ln{ \left[ \left( 1 - x \right) ^{\frac{1}{2}} \right] } \\ &= 5 \ln{ \left( x^2 + 1 \right) } - \frac{1}{2} \ln{ \left( 1 - x \right) } \end{align*}

These functions are MUCH easier to differentiate...