### Differential and Integral Calculus (21-120) — Feedback on Homework 5

Homework 5 was due on Thursday 26th September 2013 and consisted of:

- Section 3.6 Q 3, 9, 23, 39, 49

I marked Q 9, 23 and 39 (out of 3 each); a free mark was given for submitting the homework.

**Section 3.6 Q9.** This question was done well on the whole; the most common error was differentiating $\ln(5x)$ incorrectly, obtaining either $\frac{1}{5x}$ or $\frac{5}{x}$. In fact, the derivative of $\ln(5x)$ is just $\frac{1}{x}$.

**Section 3.6 Q23.** Most errors arising in this question were from people forgetting to apply the product rule and chain rule.

**Section 3.6 Q39.** People who did this question the intended way usually did it well. The question asked specifically that you should use logarithmic differentiation, in other words you start by writing $$\ln(y) = 2\ln(x^2+2) + 4\ln(x^4+4)$$ and proceed from there. Another common error was for people to get $4x$ instead of $4x^3$ as their derivative of $x^4$... be careful!

Back to course page