I want check my answer in derivatives (part 1)
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FAO OP: It is easier on us if you can use LaTeX or even plain text, if used correctly. I noticed with another post that your image host does not seem too reliable or quick.
NB: on your notation it looks like you're doing $\displaystyle \bar{y}$ which means something completely different.
In question 1 where has your denominator gone?
Question 2 - check your signs when using negative exponents. $\displaystyle \dfrac{d}{dz} -z^{-n} = nz^{-n-1}$ (either that or I cannot make out the + sign)
Question 3 - Yep
edit: you can also use wolfram to find out the answers to derivatives type in derive f(x) where f(x) is your function