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Re: What is the value of 2/(2 - sqrt2)?

by Brent@GMATPrepNow » Fri Mar 27, 2020 9:10 am

BTGModeratorVI wrote: ↑
Wed Mar 25, 2020 6:18 am
What is the value of 2/(2−√2)?

(A) 1
(B) 2
(C) 1 + √2
(D) 2 + √2
(E) 4\
Answer: D
Source: Veritas Prep

Given: 2/(2 - √2)

Our goal is to rationalize ("fix") the denominator so that it doesn't have a root.
We'll do this by multiplying the numerator and denominator by the conjugate of 2 - √2, which is 2 + √2

Numerator: 2(2 + √2) = 4 + 2√2
Denominator: (2 - √2)(2 + √2) = 4 - 2 = 2

So, 2/(2 - √2) = (4 + 2√2)/2 = 2 + √2

Answer: D

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

Re: What is the value of 2/(2 - sqrt2)?

by Brent@GMATPrepNow » Fri Mar 27, 2020 9:11 am

BTGModeratorVI wrote: ↑
Wed Mar 25, 2020 6:18 am
What is the value of 2/(2−√2)?

(A) 1
(B) 2
(C) 1 + √2
(D) 2 + √2
(E) 4\
Answer: D
Source: Veritas Prep

Another approach:

All GMAT test-takers should all memorize the following approximations:
√2 ≈ 1.4
√3 ≈ 1.7
√5 ≈ 2.2

So, 2/(2 - √2) ≈ 2/(2 - 1.4)
≈ 2/(0.6)
≈ 3.3

Now check the answer choices...
(A) 1 nope (ELIMINATE)
(B) 2 nope (ELIMINATE)
(C) 1 + √2 ≈ 1 + 1.4 ≈ 2.4 nope (ELIMINATE)
(D) 2 + √2 ≈ 2 + 1.4 ≈ 3.4 looks good. KEEP
(E) 4 nope (ELIMINATE)

Answer: D

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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Re: What is the value of 2/(2 - sqrt2)?

by Scott@TargetTestPrep » Sun Mar 29, 2020 4:07 am

BTGModeratorVI wrote: ↑
Wed Mar 25, 2020 6:18 am
What is the value of 2/(2−√2)?

(A) 1
(B) 2
(C) 1 + √2
(D) 2 + √2
(E) 4\
Answer: D
Source: Veritas Prep

When we have a radical or radical expression in the denominator, we must rationalize the denominator, in this case by multiplying the numerator and denominator by the conjugate of (2 - √2), which is (2 + √2).

2 / (2−√2) x (2+√2) / (2+√2)

Note that when the expressions in the denominator are FOILed, we obtain 4 + 2√2 - 2√2 - 2, which simplifies to 4 - 2.

2(2+√2) / (4 - 2)

2(2+√2) / 2

2 + √2

Answer: D

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