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

Redeem

gmat prep Sq roots!

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 222
Joined: Tue Mar 25, 2008 3:52 pm
Thanked: 2 times

gmat prep Sq roots!

by vinviper1 » Thu Aug 28, 2008 10:56 am
any strategy to these approximations? I knew that it would be 13 if it was the sq root of 9 instead of 10 but I then expect it to be a bit higher

Thanks!
Attachments
Q15.JPG
Top
Source: — Problem Solving |
Junior | Next Rank: 30 Posts
Posts: 18
Joined: Sat Aug 02, 2008 12:39 pm

by Shrinidhi » Thu Aug 28, 2008 1:21 pm
Apply the formula (a+b)^2 = a^2 + 2ab + b^2
Hence,

(sqrt(2) + sqrt(5))^2
= (sqrt(2))^2 + 2(sqrt(2))(sqrt(5)) + ((sqrt(5))^2
= 2 + 2(sqrt(10)) + 5
= 7 + 2(sqrt(10))

Once you get to this, you need not calculate the exact value of sqrt(10).
Sqrt(9) is 3.. so sqrt(10) would be just over 3. So, we can assume sqrt(10) as 3, since the problem asks for approximate value.

So, the equation becomes = 7 + 2(3) = 13, C it is.

Hope this helps. Please share if you have any better ideas.
Top
Senior | Next Rank: 100 Posts
Posts: 50
Joined: Tue Mar 11, 2008 6:30 am
Thanked: 6 times

by bourne159 » Thu Aug 28, 2008 2:19 pm
Since question is asking for an approximate answer something close to the rel answer is good enough.

2+5 + 2root(10)
Let's take the two cases you can check.
1. 2 + 5 + 2 root(9) = 13
2. 2 + 5 + 2 root(16) = 15

You know that root(10) is closer to root(9) than root(16)
hence 13 is the answer.
Top
Legendary Member
Posts: 661
Joined: Tue Jul 08, 2008 12:58 pm
Location: France
Thanked: 48 times

by pepeprepa » Thu Aug 28, 2008 2:57 pm
vinviper you should know square roots of 3 and 5, it will make you save time, use it as soon as you have this kind of problem and you will remember them fastly.
In this case you just add the two square roots you know and square it.
Top
Post new topic Post Reply
• Page 1 of 1