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Length of diagonal

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by gmattesttaker2 » Sat Nov 23, 2013 11:13 am
Hello,

Can you please assist with this?

The four sides of quadrilateral JKLM have equal length. What is the length of
diagonal JL?

(1) JK = 2
(2) KM = 2


OA: C


When I tried to solve this problem I assumed that JKLM is a square but I guess it is a rhombus. I was wondering how can I decide whether it is a square or a rhombus given that the question says all sides are equal.

Thanks,
Sri
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by riz_gmat » Sat Nov 23, 2013 11:50 am
Hi Sri,

Diff between Square and Rhombus is ,
for Square , diagonals are equal
while for Rhombus - Diagonals are unequal!

Coming to question...
To find : Length of diagonal JL

Option 1 : JK = 2 ( ie, all sides = 2)
It can be a square or Rhombus .
Not sufficient !

Option 2 : KM = 2 ( One diagonal = 2)
Nothing mentioned about sides to calculate the diagonals and make sure they r equal or unequal.
Not sufficient !


Both Options combined:
Sides = 2 One diagonal = 2
It cannot be a square , since if sides = 2 ,then diagonals will be 2 root 2.
Hence it is rhombus and can find the length of other diagonal using formula
4 (side)^2 = (d1)^2 + (d2)^2.
Sufficient!
Hence C.
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by [email protected] » Sat Nov 23, 2013 2:47 pm
Hi Sri,

This DS question, while a bit complex, serves as a great lesson that you can apply to ALL DS questions: be CAREFUL about your assumptions.

If the prompt wanted you to know that you were dealing with a square, then it would have used the word "square." By using the word "quadrilateral", you have to ask yourself what THAT WORD means. I know that I'm dealing with a 4-sided shape, but I'm not sure what type. With the added information about how the four sides have equal length, that narrows it down to a square or a rhombus.

DS questions put an emphasis on your "thoroughness", so make sure that you are careful to consider more than just the obvious possibilities.

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