These isssues refer to OG 11 DS problems 143 and 145 shown below

# 143

If m>0 and n>0, is (m+x)/(n+x) > m/n

(1) m<n
(2) x>0

#145

Is 1/p > r/(r^2 +2)

(1) p =r
(2) r >0

The issue I have with both these problems is that in the cae of 143 the question does not say x is not equal to -n leaving open the possiblity that the first term in the stem (m+x)/(n+x) could be undefined. I have the same issue in 145 in that the question doesn't state that p is not equal to 0 which would make the 1/p undefined. Is there a reason GMAC does not put this information in the questions?

Intriguing question, as they're usually so precise about noting those things. May just be an editing error - there are lots of typos and editing errors throughout OG, unfortunately. I'm going to forward this to our curriculum director and see if he has any ideas.

Anyway, rest assured, the test does not deal with "undefined" - so we don't need to worry about that possibility.

The GMAT folks aren’t as consistent as they ought to be on this issue. Sometimes they do write the “no denominator = 0” condition, e.g. OG 11 #127 (“In the expression above, if xn does not equal zero...”) or #139 (“if x does not equal –y, is (x-y)/(x+y) > 1?”), and sometimes they don’t, as in #143 and #145. We have yet to find a problem, however, in which they failed to put that condition and then actually *tested* that you caught its absence. In other words, #143 and #145 are graded as if the condition were in place.

Somewhat similarly, if they write “the square root of x” in some expression, they won’t always say explicitly that x is non-negative, but you can assume that they don’t write undefined expressions or ones that go outside the bounds set in the OG (e.g., the square root of a negative number is an “imaginary” number, which is outside the bounds of the GMAT).

Stacey, Christian,

Thanks for your responses.