{"id":11436,"date":"2016-05-13T21:53:33","date_gmt":"2016-05-13T21:53:33","guid":{"rendered":"http:\/\/www.manhattanprep.com\/gmat\/?p=11436"},"modified":"2020-10-28T14:49:34","modified_gmt":"2020-10-28T14:49:34","slug":"heres-why-youre-getting-geometry-problems-on-gmat-data-sufficiency-wrong","status":"publish","type":"post","link":"https:\/\/www.manhattanprep.com\/gmat\/blog\/heres-why-youre-getting-geometry-problems-on-gmat-data-sufficiency-wrong\/","title":{"rendered":"Getting Geometry Problems on GMAT Data Sufficiency Wrong?"},"content":{"rendered":"

\"ManhattanDid you know that you can attend the first session of any of our online or in-person GMAT courses absolutely free? We\u2019re not kidding! Check out our upcoming courses here<\/a>.
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I had a student recently whose Data Sufficiency (DS) accuracy was generally very high, and whose knowledge of geometry rules was solid. And yet, she was getting almost every geometry DS question wrong on practice tests!<\/p>\n

This is actually very common: students who are otherwise good at geometry and\/or DS struggle when the two things are put together.<\/p>\n

Here are the 3 main reasons that students miss geometry DS problems:<\/p>\n

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  1. They trust their eyes too much<\/u><\/strong><\/li>\n<\/ol>\n

    Consider this problem:<\/p>\n

    \"Manhattan<\/p>\n

    If we want to know the area of the circle, we need to know the radius. Since we have a square inscribed in the circle, the diameter of the circle would be the diagonal of the square:<\/p>\n

    \"Manhattan<\/p>\n

    Therefore, if we have any length of the square \u2013 side length or diagonal \u2013 we can find the diameter of the circle, and thus the area. Statement (1) gives us a side length, and statement (2) gives the diagonal, so they\u2019re both sufficient. The answer is D. Right?<\/p>\n

    Wrong.<\/p>\n

    There\u2019s a major assumption that we just fell for here: we don\u2019t know for sure that it\u2019s a square!<\/em> It looks like one, and it probably<\/em> is, but we can\u2019t assume anything. Some of the sides might be imperceptibly shorter than the others.<\/p>\n

    Just knowing one side length, as in statement (1), would not be enough to find the diameter if ABCD is not a square.<\/p>\n

    Statement (2) tells us that the line from A to C is the hypotenuse of a right triangle. By definition, the hypotenuse of any right triangle inscribed in a circle is also the diameter of the circle. If we have that diameter, we can solve for the radius and thus the area of the circle. The correct answer is B.<\/p>\n

    You have to prove it.<\/strong><\/p>\n

    Don\u2019t just rely on your eyes! Ask yourself if they\u2019re given you enough information to PROVE that the shape is what it appears to be.<\/p>\n

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    1. They don\u2019t unpack the diagram before going to the statements<\/u><\/strong><\/li>\n<\/ol>\n

      Try this problem:<\/p>\n

      \"Manhattan<\/p>\n

      You probably memorized the rule that the area of a rhombus is (diagonal1 <\/sub>\u00d7 diagonal2<\/sub>)\/2. So you might start plugging the statement information into the problem to see if you can get the length of BG and AC, CE and DF.<\/p>\n

      If that\u2019s the case, you might end up with E, which is a wrong answer.<\/p>\n

      Instead, start by asking yourself \u201cwhy<\/em> did they give me all of this given information?\u201d If all we care about is the area of each rhombus, why did they bother giving us an equilateral triangle?? Before you dive into the statements, make every inference you can:<\/p>\n