gmat prep

Expert replies

awilhelm: Master | Next Rank: 500 Posts; Posts: 117; Joined: Fri Oct 17, 2008 7:41 am

by awilhelm » Fri Jan 23, 2009 1:12 pm

[sqrt{9 + sqrt(80)} + sqrt{9 - sqrt(80)}]^2 = ?

a) 1
b) 9 - 4(sqrt5)
c) 18 - 4(sqrt5)
d) 18
e) 20

What's the best way to approach this? (x + y)^2?

Like this post

Source: Beat The GMAT — Problem Solving | Fri Jan 23, 2009 1:12 pm

DanaJ: Site Admin; Posts: 2567; Joined: Thu Jan 01, 2009 10:05 amFollowed by: 550 members; Thanked: 712 times; GMAT Score:770

by DanaJ » Fri Jan 23, 2009 1:30 pm

Let's make some notations:
sqrt(80) = a and I'll use sq instead of sqrt for easier writing.
Indeed you have to use the (x+y)^2 = x^2 + y^2 + 2xy formula
You get:
[sq(9 + a) + sq(9-a)]^2 = [sq(9+a)]^2 + [sq(9-a)]^2 + 2sq(9+a)sq(9-a). This is the equivalent of 9 + a + 9 - a + 2sq[(9+a)(9-a)] = 18 + 2sq(9^2 - a^2). Since a = sqrt(80), then a^2 = 80. So you ultimately have:

18 + 2sq(9^2 - a^2) = 18 + 2sq(81-80)=18 + 2sq(1)= 20

Answer E.

For extra info: [(9+a)(9-a)] = 9^2 - a^2 by using the formula (x-y)(x+y) = x^2 - y^2.

Like this post

awilhelm: Master | Next Rank: 500 Posts; Posts: 117; Joined: Fri Oct 17, 2008 7:41 am

by awilhelm » Fri Jan 23, 2009 1:42 pm

Very clear. Thanks!

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review