(a + b*)* =

This topic has expert replies
Master | Next Rank: 500 Posts
Posts: 151
Joined: Wed Feb 11, 2009 2:45 am
Thanked: 3 times

(a + b*)* =

by deepoe » Sun Mar 01, 2009 9:05 am
(a + b*)* =

(A) b – a
(B) a – b – 4
(C) b – a + 4
(D) a + b – 2
(E) a – b

Could someone resolve this one and explain why it is like the way you do it?


THANK YOU!!
Source: — Problem Solving |

User avatar
Site Admin
Posts: 2567
Joined: Thu Jan 01, 2009 10:05 am
Thanked: 712 times
Followed by:550 members
GMAT Score:770

by DanaJ » Sun Mar 01, 2009 10:11 am
You missed smth over here:
(a + b*)* =

Correct the problem and we'll try to help.

Master | Next Rank: 500 Posts
Posts: 151
Joined: Wed Feb 11, 2009 2:45 am
Thanked: 3 times

by deepoe » Sun Mar 01, 2009 10:19 am
this is a question at the defined functions :D

User avatar
Site Admin
Posts: 2567
Joined: Thu Jan 01, 2009 10:05 am
Thanked: 712 times
Followed by:550 members
GMAT Score:770

by DanaJ » Sun Mar 01, 2009 10:23 am
I'm sorry, I'm not familiar with it... I'm not from the US and you guys might have slightly different notations... If you would be so kind as to explain, maybe I could help...

Master | Next Rank: 500 Posts
Posts: 392
Joined: Thu Jan 15, 2009 12:52 pm
Location: New Jersey
Thanked: 76 times

by truplayer256 » Sun Mar 01, 2009 10:34 am
Is there another piece of important information that goes along with this problem? If not, then I don't think this problem is solvable.

Master | Next Rank: 500 Posts
Posts: 151
Joined: Wed Feb 11, 2009 2:45 am
Thanked: 3 times

by deepoe » Sun Mar 01, 2009 11:39 am
I found the answer in the book, but don't understand it really :$

(a + b*)* = (a + [2 – b])* = (a + 2 – b)* = 2 – (a + 2 – b) = 2 – a – 2 + b = –a + b = b – a. The answer is
(A).

User avatar
Site Admin
Posts: 2567
Joined: Thu Jan 01, 2009 10:05 am
Thanked: 712 times
Followed by:550 members
GMAT Score:770

by DanaJ » Sun Mar 01, 2009 11:46 am
The problem might be wrong, but with the info that you provided I believe that the "*" operation is defined as:
x* = 2 - x.
After defining the problem, they ask you to calculate (a + b*)*. You begin by working the stuff in the parenthesis: b* = 2 - b and you replace b* in the initial form:
(a + b*)* = (a + 2 - b)*.
Now, from this point you consider a + 2 - b as a number on its own and you apply the operation:
(a + 2 - b)* = 2 - (a + 2 - b) = 2 - a - 2 + b = b - a.

Master | Next Rank: 500 Posts
Posts: 137
Joined: Thu Jan 08, 2009 1:27 am
Thanked: 7 times

by welcome » Mon Mar 02, 2009 1:27 am
Deepoe, please check the question again. How come one put b=2-b fomr nowhere... there should a defined function as DanaJ said.
Shubham.
590 >> 630 >> 640 >> 610 >> 600 >> 640 >> 590 >> 640 >> 590 >> 590