What is the value of a^4 - b^4? (1) a^2 - b^2 = 16 (2) a + b

Oldest

Best Reply

Author

Message

TAGS:

Manager

Joined: 05 May 2005

Posts: 82

Followers: 1

Kudos [?]: 0 [0], given: 0

What is the value of a^4 - b^4? (1) a^2 - b^2 = 16 (2) a + b [#permalink]

23 Aug 2007, 17:55

00:00

Question Stats:

0% (00:00) correct

0% (00:00) wrong

based on 0 sessions

What is the value of a^4 - b^4?

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

(2) a + b = 8

Manager

Joined: 03 Sep 2006

Posts: 237

Followers: 1

Kudos [?]: 5 [0], given: 0

[#permalink]

23 Aug 2007, 19:35

Ans. is C:

(a^2-b^2)*(a^2+b^2) = (a^2+b^2)*(a-b)*(a+b)

I. Ins: we know only one part
II. Ins: But we can fugure out what is the value of a-b

Together: Suff

Manager

Joined: 18 Jun 2007

Posts: 89

Followers: 2

Kudos [?]: 0 [0], given: 0

[#permalink]

23 Aug 2007, 20:34

What is the answer for this one-

I come up with A as the answer

from one ...
a ^2 - b ^ 2 = ( a+b) ( a-b) = 16

The only values that satify this are 8 and 2 ...

you can solve for a nad b...

Manager

Joined: 08 Oct 2006

Posts: 217

Followers: 1

Kudos [?]: 5 [0], given: 0

[#permalink]

24 Aug 2007, 00:17

Sumithra Sen wrote:

What is the answer for this one-

I come up with A as the answer

from one ...
a ^2 - b ^ 2 = ( a+b) ( a-b) = 16

The only values that satify this are 8 and 2 ...

you can solve for a nad b...

I don't agree with this.
We cannot assume that the respective values will be a-b= 2
and a+b= 8.

a and b can be any values here. one of them could be negative, or a fraction or any combination.
Also if b is negative it will result in a-b actually greater than a+ b, impacting the values of a and B and thus the values of a^4-b^4.

So I go with C.

Intern

Joined: 30 Jun 2007

Posts: 7

Followers: 0

Kudos [?]: 0 [0], given: 0

[#permalink]

27 Aug 2007, 14:05

Sumithra Sen wrote:

What is the answer for this one-

I come up with A as the answer

from one ...
a ^2 - b ^ 2 = ( a+b) ( a-b) = 16

The only values that satify this are 8 and 2 ...

you can solve for a nad b...

Incorrect assumption.

a= 8, b=2 is one solution. So is a=4, b=0. In both cases, a^4-b^4 will give differerent results. St1 is insuff.

S2 - a+b =8. Insuff.

S1+S2: a+b = 8 and a-b=2. Suff.

Current Student

Joined: 28 Dec 2004

Posts: 3437

Location: New York City

Schools: Wharton'11 HBS'12

Followers: 11

Kudos [?]: 135 [0], given: 2

[#permalink]

27 Aug 2007, 14:23

this is C..

a^4-b^4=(a^2-b^2)(a^2+b^2)

1) a^2-b^2=16

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

insuff

2) a+b=8...insuff

1+2

(a-b)(8)=16

then a-b=2

you get 2 simeltaneous equations

a-b=2
a+b=8

2a=10

a=5

5+3=8
5-3=2

now we can soleve 5^4-3^4

Joined: 10 Jun 2007

Posts: 1478

Followers: 5

Kudos [?]: 73 [0], given: 0

Re: Algebra - Arithmetic (DS) [#permalink]

27 Aug 2007, 14:42

above720 wrote:

What is the value of a^4 - b^4?

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

(2) a + b = 8

C for me

You know that a^4 - b^4 = (a^2 - b^2) * (a^2 + b^2)
(1) only tell a^2-b^2
Insufficient

(2) Doesn't tell anything.

Together:
(a+b)*(a-b) = 16
Plug in
a-b = 2
a+b = 8
Solve for a and b, get a=5, b=3
SUFFICIENT.

Re: Algebra - Arithmetic (DS) [#permalink] 27 Aug 2007, 14:42

	Similar topics	Author	Replies	Last post
Similar Topics:	What is the value of a^4-b^4? 1) a^2 - b^2 = 16 2) a + b = 8	Questor	4	23 Dec 2004, 00:32
	What is the value of a^4 - b^4 ? I - a^2 - b^2 = 16 II	tobiastt	1	25 Nov 2005, 09:43
	What is the value of a^4 - b^4 ? 1) a^2 - b^2 = 16 2) a+b =	dorilee6	2	22 Dec 2006, 09:20
	What is the value of a^4 - b^4? 1) a^2 - b^2 = 16 2) a + b =	jimmyjamesdonkey	3	25 Nov 2007, 16:06
	If xy=1, what is the value of / a) 2 b) 4 c) 8 d) 16 e) 32	chouchou	10	07 Jul 2008, 17:55

What is the value of a^4 - b^4? (1) a^2 - b^2 = 16 (2) a + b

Main navigation

GMAT Resources

Partners

MBA Resources

GMAT ® is a registered trademark of the Graduate Management Admission Council ® (GMAC ®). GMAT Club's website has not been reviewed or endorsed by GMAC.

GMAT Courses

Admissions Consulting

What is the value of a^4 - b^4? (1) a^2 - b^2 = 16 (2) a + b

What is the value of a^4 - b^4? (1) a^2 - b^2 = 16 (2) a + b

Main navigation

GMAT Resources

Partners

MBA Resources