It is currently 21 Feb 2018, 23:10

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# What is the value of a^4 – b^4 ?

Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 43862
What is the value of a^4 – b^4 ? [#permalink]

### Show Tags

24 Sep 2017, 23:03
Expert's post
1
This post was
BOOKMARKED
00:00

Difficulty:

55% (hard)

Question Stats:

62% (01:34) correct 38% (01:39) wrong based on 26 sessions

### HideShow timer Statistics

What is the value of $$a^4 – b^4$$ ?

(1) $$(a – b)(a + b)^3 = 10$$

(2) $$ab(a^2 – b^2) = 5$$
[Reveal] Spoiler: OA

_________________
Intern
Joined: 27 Feb 2015
Posts: 8
Re: What is the value of a^4 – b^4 ? [#permalink]

### Show Tags

24 Sep 2017, 23:14
a^4 - b^4 =( a^2- b^2) (a^2 + b^2)
a^2- b^2 = (a -b)(a+b)
so a^4 - b^4 = (a -b)(a+b)(a^2 + b^2) Stat 1 NS

Stat 2. NS

Ans C

Sent from my SM-G935F using GMAT Club Forum mobile app

Last edited by Ejiroghene on 25 Sep 2017, 02:12, edited 1 time in total.
Senior Manager
Joined: 02 Jul 2017
Posts: 288
GMAT 1: 730 Q50 V38
Re: What is the value of a^4 – b^4 ? [#permalink]

### Show Tags

25 Sep 2017, 00:32
Ejiroghene wrote:
a^4 - b^4 =( a^2- b^2) (a^2 + b^2)
a^2- b^2 = (a -b)(a+b)
so a^4 - b^4 = (a -b)(a+b)(a^2 + b^2) which is = (a -b)(a+b)^3 = 10. Stat 1 Suff

Stat 2. NS

Ans A

Sent from my SM-G935F using GMAT Club Forum mobile app

Mistake : $$a^4 - b^4 = (a -b)(a+b)(a^2 + b^2)$$ is "Not" equal to $$(a -b)(a+b)^3$$

=> $$a^4 - b^4 = (a-b)(a+b)(a^2+b^2) = (a-b)(a+b)(a^2+b^2+2ab-2ab) = (a-b)(a+b)[(a+b)^2 -2ab] = (a-b)(a+b)^3 - 2ab(a-b)(a+b)$$

$$= (a-b)(a+b)^3 - 2ab(a^2-b^2)$$

Statement 1 and statement 2 alone are not sufficient to solve it.
By combining 1 and 2 we can calculate value of $$a^4-b^4$$ by putting values from 1 and 2 in above mentioned equation

Re: What is the value of a^4 – b^4 ?   [#permalink] 25 Sep 2017, 00:32
Display posts from previous: Sort by