BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

If x >= 0 and x=root(8xy-16y^2), then, in terms of y, x=?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Moderator
Posts: 2599
Joined: Sun Oct 29, 2017 2:08 pm
Followed by:2 members

If x >= 0 and x=root(8xy-16y^2), then, in terms of y, x=?

by AAPL » Tue Dec 05, 2017 10:46 am
$$If\ \ x\ge0\ \ and\ \ \ x=\sqrt{8xy-16y^2}$$ then, in terms of y, x=?

A. -4y
B. 4/y
C. y
D. 4y
E. 4y^2

The OA is D.

Experts, can help me with this PS question please? Thanks.

I know that I should get the x value of the expression,

$$x^2-8xy+16y^2=0$$

But, I can't determinate it, I should factorize the expression but I can't get the correct answer. I don't know where is my error.
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 555
Joined: Wed Oct 04, 2017 4:18 pm
Thanked: 180 times
Followed by:12 members

by EconomistGMATTutor » Tue Dec 05, 2017 10:55 am
Hello AAPL.

Let's take a look here.

Your first approach is correct. From $$x=\sqrt{8xy-16y^2}$$ we get $$x^2-8xy+16y^2=0.$$ Now, using the quadratic formula we get $$x_{1,2}=\frac{-\left(-8y\right)\pm\sqrt{\left(-8y\right)^2-4\cdot1\cdot16y^2}}{2}=\frac{8y\pm\sqrt{64y^2-64y^2}}{2}=\frac{8y}{2}=4y.$$ So, the correct answer is D.

I hope this explanation may help you to clarify your doubt.

I'm available if you'd like a follow up.

Regards.
GMAT Prep From The Economist
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.

Image
Top
GMAT Instructor
Posts: 2630
Joined: Wed Sep 12, 2012 3:32 pm
Location: East Bay all the way
Thanked: 625 times
Followed by:119 members
GMAT Score:780

by Matt@VeritasPrep » Tue Dec 05, 2017 6:30 pm
A little easier:

x² - 8xy + 16y² = 0

(x - 4y)² = 0

x = 4y
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 8086
Joined: Sat Apr 25, 2015 10:56 am
Location: Los Angeles, CA
Thanked: 43 times
Followed by:29 members

by Scott@TargetTestPrep » Fri Oct 11, 2019 2:45 pm
AAPL wrote:$$If\ \ x\ge0\ \ and\ \ \ x=\sqrt{8xy-16y^2}$$ then, in terms of y, x=?

A. -4y
B. 4/y
C. y
D. 4y
E. 4y^2

The OA is D.

Squaring the equation, we have:

x^2 = 8xy - 16y^2

x^2 - 8xy + 16y^2 = 0

(x - 4y)(x - 4y) = 0

x - 4y = 0

x = 4y

Answer: D

Scott Woodbury-Stewart
Founder and CEO
[email protected]

Image

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews

ImageImage
Top
Post new topic Post Reply
• Page 1 of 1