Hints for Exercises from Section 5.2 =MATH 3000 Kawai [5.7] Prove That

Hints for Exercises from Section 5.2 =MATH 3000 Kawai [5.7] Prove That

Hints for Exercises from Section 5.2 –MATH 3000 Kawai [5.7] Prove that there is NO largest negative rational number. BWOC, suppose x is largest negative rational number. Let y = x=2: It is also negative and rational. If x < 0; then 1 1 < 1 x > 1 x; 2 ) 2 since we multiplied both sides by a negative number. Both sides are negative. x x < = y: ( ) 2 )( This contradicts that x is the largest negative rational number. [5.8] Prove that there is no smallest positive irrational number. Same as [5.7], but we need to show that if x is irrational, then x=2 is also irrational. EASY. Contrapositive! x If Q; then x Q: 2 2 2 We had a lemma that covers this. [5.9] BWOC, suppose 200 can be written as odd + even + even: ( ) )( [5.10] If a and b are odd, then 4 - a2 + b2 : Here’swhy indirect proof is so awesome. BWOC, we start with 4 a2 + b2 a2 + b2 = 4k: j ) If a and b are odd, then we use the usual representatives: a = 2k1 + 1; b = 2k2 + 1: This gives us: 2 2 (2k1 + 1) + (2k2 + 1) = 4k 2 2 4k1 + 4k1 + 4k2 + 4k2 + 2 = 4k Since this is a derived equality, we can manipulate both sides! 2 2 2 2 2 = 4k 4k 4k1 4k 4k2 = 4 k k k1 k k2 1 2 1 2 2 2 By closure, let m = k k k1 k k2; and this gives us: 1 2 2 = 4m 4 2: ( ) ) j )( [5.11] This should make sense. If a; b Z+ (it doesn’tmake sense for either of them to be negative or zero), and a 2; 2 then a - b or a - (b + 1) : Clearly, we will contradict a dividing BOTH of them. Hint: (( P ) ( Q)) P Q: _ , ^ 1 [5.13] Prove that the product of an irrational number and a nonzero rational number is irrational. Let our original irrational number be x and our original nonzero rational number be s: BWOC, suppose their product is rational, t: xs = t Divide both sides by s: t x = s The right side is rational divided by rational, and this must be rational. The left side is irrational. ( ) )( [5.19] Let S = p + qp2; p; q Q and T = r + sp3; r; s Q : 2 2 Clearly, if we …x q = 0; then we have a subset of S; and that subset is all of Q: The same happens when we …x s = 0 in T: Prove that S T = Q: \ Tricky: BWOC, suppose S T = Q: Thus, there must be an element in both S and T which is not rational. \ 6 [5.20] Prove that if x; y R+; then px + y = px + py: 2 6 BWOC, suppose 2 2 px + y = px + py px + y = px + py ) x + y = x + 2pxy + y 2pxy = 0 pxy = 0 xy = 0: ) ) ) What did we contradict? 2.

View Full Text

Details

  • File Type
    pdf
  • Upload Time
    -
  • Content Languages
    English
  • Upload User
    Anonymous/Not logged-in
  • File Pages
    2 Page
  • File Size
    -

Download

Channel Download Status
Express Download Enable

Copyright

We respect the copyrights and intellectual property rights of all users. All uploaded documents are either original works of the uploader or authorized works of the rightful owners.

  • Not to be reproduced or distributed without explicit permission.
  • Not used for commercial purposes outside of approved use cases.
  • Not used to infringe on the rights of the original creators.
  • If you believe any content infringes your copyright, please contact us immediately.

Support

For help with questions, suggestions, or problems, please contact us