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# (5+1/5)^2-(5-1/5)^2=?

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 9006
GMAT 1: 760 Q51 V42
GPA: 3.82

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27 Apr 2017, 00:53
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Difficulty:

15% (low)

Question Stats:

86% (01:09) correct 14% (02:01) wrong based on 59 sessions

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$$(5+\frac{1}{5})^2-(5-\frac{1}{5})^2$$=?

A. 2
B. 3
C. 4
D. 5
E. 9

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MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Senior Manager Joined: 02 Mar 2017 Posts: 252 Location: India Concentration: Finance, Marketing Re: (5+1/5)^2-(5-1/5)^2=? [#permalink] ### Show Tags 27 Apr 2017, 00:58 1 (a+b)^2 - (a-b)^2 = 4ab Here ab = 1 ------> 4 ------->C Senior Manager Joined: 24 Apr 2016 Posts: 306 Re: (5+1/5)^2-(5-1/5)^2=? [#permalink] ### Show Tags 27 Apr 2017, 01:01 (5+1/5)^2−(5−1/5)^2 Using the formula a^2 - b^2 = (a+b)*(a-b) Above can be written as (5+1/5+5-1/5)*(5+1/5-5+1/5) = (5+5)*(1/5+1/5) = 10*2/5 = 4 Answer is C Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 9006 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: (5+1/5)^2-(5-1/5)^2=? [#permalink] ### Show Tags 30 Apr 2017, 17:12 ==> Since $$a^2-b^2=(a-b)(a+b)$$, you get $$(5+\frac{1}{5})^2-(5-\frac{1}{5})^2=[(5+\frac{1}{5})-(5-\frac{1}{5})][(5+\frac{1}{5})+(5-\frac{1}{5})]=(\frac{2}{5})(10)=4$$. The answer is C. Answer: C _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$79 for 1 month Online Course"
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06 May 2017, 16:25
MathRevolution wrote:
$$(5+\frac{1}{5})^2-(5-\frac{1}{5})^2$$=?

A. 2
B. 3
C. 4
D. 5
E. 9

Let’s simplify the given expression:

(5 + 1/5)^2 - (5 - 1/5)^2

(5 1/5)^2 - (4 4/5)^2

(26/5)^2 - (24/5)^2

(26/5 + 24/5)(26/5 - 24/5)

(50/5)(2/5) = 10 x 2/5 = 4

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Re: (5+1/5)^2-(5-1/5)^2=?   [#permalink] 06 May 2017, 16:25