### Differential and Integral Calculus (21-120) — Feedback on Homework 9

Homework 9 was due on Thursday 24th October 2013 and consisted of:

- Section 4.5 Q 42, 52
- Section 4.7 Q 14, 28

I marked 4.5/42 (out of 4) and 4.7/14 (out of 4); two free marks were given for submitting the homework.

**Section 4.5 Q42.** I graded this problem very leniently; provided you'd found the derivatives correctly and identified the correct root, local maximum, infinite behaviour and inflection point, you got all the marks. Graphs could have been better labelled, though!

**Section 4.7 Q14.** This was done well by almost everyone. The most common error was failing to check that the obtained value is a *minimum* rather than a maximum value. Another common error was using the wrong formula for the surface area of an open box: many people solved the problem for a *closed* box!

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