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

Redeem

(Gmat) Faster Method needed

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Legendary Member
Posts: 2134
Joined: Mon Oct 20, 2008 11:26 pm
Thanked: 237 times
Followed by:25 members
GMAT Score:730

(Gmat) Faster Method needed

by logitech » Fri Nov 28, 2008 1:03 am
If q, r, and s are consecutive even integers and q < r < s, which of the following CANNOT be the value of s2 – r2 – q2?


-20
0
8
12
16

OA [spoiler]8[/spoiler]
LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"
Top
Source: — Problem Solving |
Legendary Member
Posts: 829
Joined: Mon Jul 07, 2008 10:09 pm
Location: INDIA
Thanked: 84 times
Followed by:3 members

Re: (Gmat) Faster Method needed

by sudhir3127 » Fri Nov 28, 2008 4:46 am
logitech wrote:If q, r, and s are consecutive even integers and q < r < s, which of the following CANNOT be the value of s2 – r2 – q2?


-20
0
8
12
16

OA [spoiler]8[/spoiler]
correct me if i am wrong .. but i think the question shud read as
S^2- r^2-q^2...

i think plugging in would be the only approach .. though u can form a quadratic equation as well ..

first approach ..

with q=-2, r=0, and s=2. Plugging that in to the given equation gives us 0

q=2, r=4, s=6 gives us 16

q=-4, r=-2, s=0 gives us -20

i mean u can stop @ this because if see. we have gone from 0 to 12 and missed 8 . so it shud be 8.

second appraoch .

let the 3 numbers ve X, X+2 , X+4

(x+4)^2 - (x+2)^2 - x^2
= x^2 + 8x + 16 - x^2 - 4x - 4 - x^2
= -x^2 + 4x + 12

on C doesnt hold good..

hope that helps...
Top
Senior | Next Rank: 100 Posts
Posts: 47
Joined: Wed Oct 08, 2008 11:29 am
Location: Toronto, Ontario
Thanked: 5 times

by anayeri » Fri Nov 28, 2008 10:54 am
I don't get the equation method. I understand how you got as far as you did. I would even got a step further, and say that:

=-x^2+4x_12
=-(x-6)(x+2)

but so what? what do I do with that information?
Top
User avatar
Legendary Member
Posts: 2134
Joined: Mon Oct 20, 2008 11:26 pm
Thanked: 237 times
Followed by:25 members
GMAT Score:730

by logitech » Fri Nov 28, 2008 1:45 pm
anayeri wrote:I don't get the equation method. I understand how you got as far as you did. I would even got a step further, and say that:

=-x^2+4x-12
=-(x-6)(x+2)

but so what? what do I do with that information?
and you try the answer choices: -20,0,8,12,16

For example:

-x^2+4x-12 = 8
LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"
Top
Post new topic Post Reply
• Page 1 of 1