Second derivative

1. Feb 20, 2012

Rasalhague

http://www.math.northwestern.edu/courses/placement/220_Self_Placement.pdf [Broken]

Question 7 here involves a function with the rule f(x) = 3x5-5x3. I computed the 2nd derivative as f''(x) = 60x3-30x (Mathematica agrees.), giving inflection points for f at -1/sqrt(2), 0, 1/sqrt(2). But the answer given in the PDF is 4x(4x2 - 3), whence they conclude the inflection points are -sqrt(3)/2, 0, sqrt(3)/2. Is this a mistake, or have I overlooked something?

Last edited by a moderator: May 5, 2017
2. Feb 21, 2012

Norwegian

You are right, and that is btw not the only mistake in that pdf.

3. Feb 21, 2012

Rasalhague

Thanks for confirming that, Norwegian. Could you tell me any others, or do you know if there's a list of errata anywhere online?

4. Feb 21, 2012

Norwegian

1d, derivative of ln(..) seems wrong, probably more mistakes too

5. Feb 21, 2012

Rasalhague

Oh yes, wow, so it is! The derivative of ln(2t3-1) is 6t2/(2t3-1), not 6t2/(ln(2t3-1)). When I was checking my answers for those simple ones at the beginning and saw they had something slightly different for 1d, I just assumed it was a typo on my part and didn't look that closely.