My Math Forum Need help with 3exam questions ..

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 February 1st, 2011, 02:05 PM #1 Newbie   Joined: Sep 2010 Posts: 24 Thanks: 0 Need help with 3exam questions .. I need these three questions answered please :$1.Is there a number such that the produckt of its digits is equal with 465 2.Prove that a set of all positiv integers divisble by 5 is a countable set 3. Prove that an empty set is a subset of each other set (Sry I didnt know where to post it) THANK YOU! February 1st, 2011, 02:45 PM #2 Global Moderator Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms Re: Need help with 3exam questions .. Quote:  Originally Posted by BackY 1.Is there a number such that the produckt of its digits is equal with 465 2.Prove that a set of all positiv integers divisble by 5 is a countable set 3. Prove that an empty set is a subset of each other set 1. Looks at the prime factorization of 465. 2. Use the function n = 5m to show that there is a multiple of 5 for every integer. 3. Use the definition of subset  February 1st, 2011, 02:55 PM #3 Newbie Joined: Sep 2010 Posts: 24 Thanks: 0 Re: Need help with 3exam questions .. I need them solved step by step brother :$ I'm so sorry, My friend needs them for tomorrow
February 1st, 2011, 03:27 PM   #4
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Re: Need help with 3exam questions ..

Quote:
 Originally Posted by BackY I need them solved step by step brother :\$ I'm so sorry, My friend needs them for tomorrow
Step 1: Understand the problem
Step 2: Devise a plan
Step 3: Carry out the plan
Step 4: Look back (check)

If your plan is to get us to do your ("friend's") work, then you need a new plan.

The product of (some numbers) = 465. What is the prime factorization of 465? You should be able to figure this out.
What is meant by a "digit"? Can a digit be negative? Can 37 be a digit?

Empty set subset of every (other) set.

What does it mean for A to be a subset of B?
You must have a definition! By "must", I mean "I'm positive that you have enough information to work with".

A subset B <=> (x in A => x in B)
Replace A with the empty set, and use what you know about the empty set.
You should have a false hypothesis. Google that if you don't know what that means.

February 2nd, 2011, 08:02 AM   #5
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Re: Need help with 3exam questions ..

Quote:
 Originally Posted by BackY My friend needs them for tomorrow
I guess 'your friend''s going to get the problems wrong, then. If you have the friend post here we can help, but I'm not going to do your homework for you.

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