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corgilover
Joined: 24 Sep 2018
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GMAT 1: 680 Q45 V38
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24 Dec 2018, 10:50
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If cd > 0, does (c-2)(d-2) = 4?
(1) 2c + 2d = cd
(2) c = d
(1) 2c + 2d = cd
(2) c = d
I'm having trouble understanding why item (2) isn't (also) sufficient for answering this question, if anyone can shed light I'd appreciate it!
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24 Dec 2018, 11:09
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corgigal
If cd > 0, does (c-2)(d-2) = 4?
(1) 2c + 2d = cd
(2) c = d
(1) 2c + 2d = cd
(2) c = d
I'm having trouble understanding why item (2) isn't (also) sufficient for answering this question, if anyone can shed light I'd appreciate it!
If cd > 0, does (c-2)(d-2) = 4?
Does (c-2)(d-2) = 4 ?
Does cd - 2c - 2d + 4 = 4 ?
Does 2c + 2d = cd ?
(1) 2c + 2d = cd. Directly answers the question. Sufficient.
(2) c = d. The question becomes: does (c-2)(c-2) = 4 ? --> does (c - 2)^2 = 4 --> does c = 0 or c = 4. We don't know that, so this statement is not sufficient.
Answer: A.
corgilover
Joined: 24 Sep 2018
Last visit: 28 Mar 2019
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Location: United States (FL)
GMAT 1: 680 Q45 V38
GRE 1: Q160 V161
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24 Dec 2018, 11:45
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Bunuel
(2) c = d. The question becomes: does (c-2)(c-2) = 4 ? --> does c^2 = 8. We don't know that, so this statement is not sufficient.
Answer: A.
Thanks for answering Bunuel but here's my issue, and if my arithmetic is to blame please point it out, but shouldn't (c-2)(c-2) become:
C(squared) - 2C - 2C + 4 = 4
C(squared) - 4C = 0
C(squared) = 4C
C = 4?
(edit for clarification) - And if we know C = 4, and C and D are equal, then if we plug that into the question then we can answer whether it is equal to 4 or not (the answer being yes), no?
