og ds 137

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simba12123: Senior | Next Rank: 100 Posts; Posts: 94; Joined: Tue Oct 14, 2008 1:05 pm

og ds 137

by simba12123 » Sat Oct 25, 2008 6:34 pm

what is the value of (a^4) - (b^4)?

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

2. a=b=8

qa is c

however cant see to get the meaning of (a^2)+(b^2)

Quote

Source: Beat The GMAT — Data Sufficiency | Sat Oct 25, 2008 6:34 pm

dally_gmat: Senior | Next Rank: 100 Posts; Posts: 31; Joined: Mon Sep 15, 2008 6:16 pm; Thanked: 3 times

Re: og ds 137

by dally_gmat » Sat Oct 25, 2008 7:32 pm

2nd Statement says a= b = 8, So we know for sure a^4 = 8^4 = B^4 therefore b should be sufficient.

1st statement does not answer the question, hence insufficient.

Where I am going wrong?

simba12123 wrote:what is the value of (a^4) - (b^4)?

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

2. a=b=8

qa is c

however cant see to get the meaning of (a^2)+(b^2)

Quote

rohangupta83: Legendary Member; Posts: 541; Joined: Thu May 31, 2007 6:44 pm; Location: UK; Thanked: 21 times; Followed by:3 members; GMAT Score:680

by rohangupta83 » Sun Oct 26, 2008 1:46 am

i concur with dally_gmat

second statement should be sufficient

a^4 - b^4 upon simplifying gives us

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

a=b=8

will give us
(a-b)(a+b)(a^2+b^2) = 0

imo B

Quote

reacher: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Thu May 22, 2008 5:45 am; Thanked: 1 times

Re: og ds 137

by reacher » Sun Oct 26, 2008 5:55 am

simba12123 wrote:what is the value of (a^4) - (b^4)?

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

2. a=b=8

qa is c

however cant see to get the meaning of (a^2)+(b^2)

You seem to have changed the statement 2 of the question simba. Statement 2 actually reads " a+b=8 " and with this statement one can solve only a part of the answer and thus combining the statements gets the answer C.