Venn Diagram problem (Set Theory)

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Homework Help Overview

The discussion revolves around a problem in set theory involving Venn diagrams. The universal set is defined as having 40 elements, with Set A containing 20 elements and Set B containing 17 elements. The relationship between the intersection of Sets A and B is given by the equation n(A∩B) = 1/2 n(A'∩B'). Participants are exploring how to determine the value of n(A∩B) based on these parameters.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the implications of the equation and the definitions of the sets involved. There are attempts to clarify the meaning of the denominator in the equation and the significance of the 1/2 factor. Some participants express confusion about the calculations and the logical flow of reasoning, particularly regarding the number of elements outside the sets.

Discussion Status

The discussion is ongoing, with participants providing hints and suggestions for approaching the problem. Some have proposed using Venn diagrams to visualize the relationships between the sets, while others are attempting to set up simultaneous equations to solve for the unknowns. There is a recognition of the need for clearer definitions and expressions in the problem setup.

Contextual Notes

Participants note that the original assumptions about the number of elements outside the sets may not hold true in all cases. There is also mention of the stress related to homework deadlines, which may be influencing the clarity of reasoning.

  • #31
Ascleipus said:
This means that the yellow is 2x and the red is 17-x
and then i assume i use simultaneous equations to solve it, do tell me if I'm on the right track, but please no answer^^

Don't use 2x for the yellow. Use the fact that it all adds up to 40 to figure out how many are in the yellow.
 
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  • #32
but in order to figure out the yellow i need the intersection don't I?
40-(37-x) must be more appropriate then if i am to use that it adds up to 40
 
  • #33
it seemed reasonable to do 40-(37-x)=(37-x)/2

if the yellow is 40-(37-x) and the union of the two sets is 37-x if divide that by two then i should have a functioning equation, right?

however that just solves x as being 10.333...
 
  • #34
Ascleipus said:
so a∩b=3?

Yes!

but how did you derive the equation a'∩b'=(1/2)(40-(a+b+a∩b))? I would understand if the 1/2 wasn't there, but if the 1/2 isn't there then the whole method doesn't work

a'nb' does not equal (1/2)(40-(a+b+a∩b))

a'nb' does equal (40-(a+b+a∩b))

Since anb=(1/2)a'nb' ,

anb= (1/2)(40-(a+b+a∩b))
 
  • #35
of course! thanks so much after all that i miss that 1 detail haha that's 2 problems you've helped me with, thank you to everyone else that has attempted to help me as well much appreciated everyone! physics forums is brilliant, hope i can someday help others like you have helped me :)
 
  • #36
LCKurtz said:
Don't use 2x for the yellow. Use the fact that it all adds up to 40 to figure out how many are in the yellow.

Ascleipus said:
but in order to figure out the yellow i need the intersection don't I?
40-(37-x) must be more appropriate then if i am to use that it adds up to 40

Sorry, I wrote that in a hurry. Go ahead and use 2x then just add them all up:

x + (17-x) + (20-x) + 2x = 40.

That's all there is to it.
 

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