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

Redeem

a^2 - B^2

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 85
Joined: Mon Dec 08, 2008 12:09 am

a^2 - B^2

by tj123 » Thu Jul 16, 2009 11:26 pm
what is the value of a^4 - b^4 = ?

1) a^2 - b^2 = 16
2) a+b= 8

I chose A because I thought you could raise everything by 2 like this:

(a^2 )^2- (b^2)^2 = (16)^2

which = a^4- b^4 = 256

But OA is c
please expalin why my logic is wrong[/spoiler]
Top
Source: — Data Sufficiency |
Master | Next Rank: 500 Posts
Posts: 322
Joined: Fri Mar 27, 2009 3:56 pm
Thanked: 24 times
GMAT Score:710

by mike22629 » Fri Jul 17, 2009 3:53 am
A.)
a^2 - b^2 = 16

Simplify:
(a+b)(a-b) = 16

Ins.

B.)
(a+b) = 8

Ins.

Both together:
Since (a+b) = 8, (a-b) = 2

solve for a and b:
a = 5
b = 3

Plug in to find answer at top

OA C.
Top
Master | Next Rank: 500 Posts
Posts: 322
Joined: Fri Mar 27, 2009 3:56 pm
Thanked: 24 times
GMAT Score:710

by mike22629 » Fri Jul 17, 2009 3:55 am
P.S.

You can not raise each term like the way you did

For example:

2 + 4 = 6

2^2 + 4^2 does not equal 6^2

(2+4)^2 = 6^2
Top
Post new topic Post Reply
• Page 1 of 1