home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Multimedia Geometry
/
geometry-3.5.iso
/
GEOMETRY
/
CHAPTER2.2Y
< prev
next >
Wrap
Text File
|
1995-04-21
|
6KB
|
152 lines
à 2.2èDeductive Proçs from Algebra
äèPlease complete ê followïg deductive proçs from
algebra.
â
è
èèèèèèèèèèèèè Please see Details.
éS In ê last section we looked at discoverïg patterns usïg ï-
ductive reasonïg.èIn this section we will use deductive reasonïg ë
prove that some observed patterns ï algebra are universally true.èWe
have used algebra earlier ï this program for examples ç undefïed
terms, defïitions, å axioms.èNow we will use it for examples ç de-
ductive proçs.èHowever, before we write deductive algebraic proçs, we
should make a more complete list ç ê axioms that are assumed ï that
maêmatical system.
èèèèèèèèèèèèèèèèèAXIOMS
è For all real numbers a, b, å c:
1.èThe sum ç two real numbersèèèèClosure axiom for additionèèèè
èè is a real numberèèèèè
2.èa + (b + c) = (a + b) + cèèèèèAssociative axiom for addition
3.èa + b = b + aèèèèèèèèèèèCommutative axiom for addition
4.èa + 0 = aèèèèèèèèèèèèèAdditive identity
5.èa + (-a) = 0èèèèèèèèèèè Additive ïverse
6.èThe product ç two real numbersèèClosure axiom for multiplication
èè is a real numberèèèèèèèèè
7.èa(b·c) = (ab)cèèèèèèèèèè Associative axiom for
èèèèèèèèèèèèèèèèèèèèè multiplication
8.èa·b = b·aèèèèèèèèèèèèèCommutative axiom for
èèèèèèèèèèèèèèèèèèèèè multiplication
9.èa·1 = aèèèèèèèèèèèèèèMultiplicative identity
10.èa·1/a = 1èèèèèèèèèèèèèMultiplicative ïverseèèèèèèèèèè
11.èa(b + c) = a·b + acèèèèèèèèDistributive axiom
12.èa = aèèèèèèèèèèèèèèèReflexive axiom
13.èIf a = b, ên b = aèèèèèèè Symmetric axiom
14.èIf a = b å b = c, ên a = cèè Transitive axiom
15.èIf a = b, ên you canèèèèèè Substitution axiom
èè substitute a for b
16.èEiêr a = b, a > b, or a < cèèèTrichoëmy axiom
17.èIf a = b, ên a + c = b + cèèè Addition axiom for equations
18.èIf a = b, ên ac = bcèèèèèè Multiplication axiom for
èèèèèèèèèèèèèèèèèèèèè equations
19.èIf a < b, ên a + c < b + cèèè Addition axiom for ïequalities
20.èIf a < b å c > 0, ên ac < bcè Multiplication axiom forèèèèèèèèèèèèèèèèèèèè
èè If a < b å c < 0, ên ac > bcèèèïequalities
Theorem:èIf a = b å c = d, ên a + c = b + d.
Proç:èèè Statementèèèèèèèè Reason
èèèèèè 1. a = bèèèèèèèèè1. Givenèèè
èèèèèè 2. c = dèèèèèèèèè2. Given
èèèèèè 3. a + c = b + cèèèèè3. Addition axiom for equations
Conclusion:è4. a + c = b + dèèèèè4. Substitution ç d for c
èèThe above deductive proç establishes that equals can always be
added ë equals.èThe followïg example shows how ë prove somethïg
is not true by counterexample.èIf you believe a given statement is
not universally true, all you have ë do ë prove that it is untrue
is ë fïd at least one numerical counterexample where it fails.èTo
show that a - b = b - a is not universally true, we just have ë fïd
one numerical example where it fails.
èèèèLet's try a = 5 å b = 8.èèèèèèèa - b = b - a
èèèèèèèèèèèèèèèèèèèèèèèè5 - 8 = 8 - 5
èèèèèèèèèèèèèèèèèèèèèèèèè -3 = 3
è This contradiction means that a - b = b - a is not universally true.
1
è Please prove ê followïg statement is universally true by deductive
proç, or show that it is not universally true by counterexample.
èèèèèèèèèIf a = b å c = d, ên ac = bd.
èèèA) True by deductive proçèèèB) Not true by counterexampleè
üèèèèèèIf a = b å c = d, ên ac = bd.
èProç:èè StatementèèèèèèReason
èèèèèè 1. a = bèèèèèè 1. Given
èèèèèè 2. c = dèèèèèè 2. Given
èèèèèè 3. a·c = b·cèèèè 3. Multiplication axiom for equations
Conclusion: 4. ac = bdèèèèè 4. Substitution ç d for cè
Ç A
2
è Please prove ê followïg statement is universally true by deductive
proç, or show that it is not universally true by counterexample.
èèèèèèIf a å b are real numbers, ên a ÷ b = b ÷ a.
èèèA) True by deductive proçèèèB) Not true by counterexampleè
üèèèèIf a å b are real numbers, ên a ÷ b = b ÷ a.
èTo show that a ÷ b = b ÷ a is not universally true, we just have ë
fïd one numerical counterexample where it fails.
èèè Let's try a = 4 å b = 2.èèèèè a ÷ b = b ÷ a
èèèèèèèèèèèèèèèèèèèèèè4 ÷ 2 = 2 ÷ 4
èèèèèèèèèèèèèèèèèèèèèèèè2 = 1/2
This contradiction implies that a ÷ b = b ÷ a is not universally true.èèèèèèèèèèèèèèèèè
Ç Bè
3
è Please prove ê followïg statement is universally true by deductive
proç, or show that it is not universally true by counterexample.
èèèèèèè If a = b å c = d, ên a - c = b - d.
èèèA) True by deductive proçèèèB) Not true by counterexampleè
üèèèèè If a = b å c = d, ên a - c = b - d.
èè Proç:èStatementèèèèèèèèè Reasonè
èèèèèè 1. a = bèèèèèèèèèè1. Given
èèèèèè 2. c = dèèèèèèèèèè2. Given
èèèèèè 3. a + (-c) = b + (-c)èèè3. Addition axiom for equationsèèèèèèèèèè
èèèèèè 4. a - c = b - cèèèèèè4. Defïition ç subtractionèèèèèèèèèèè
Conclusion: 5. a - c = b - dèèèèèè5. Substitution ç d for cèèèèè
Ç Aè
4
è Please prove ê followïg statement is universally true by deductive
proç, or show that it is not universally true by counterexample.
èèIf a, b, å c are real numbers, ên a + b·c = (a + b)·(a + c).
èèèA) True by deductive proçèèèB) Not true by counterexampleè
ü To show that a + b·c = (a + b)·(a + c) is not universally true,èèèèè
we just have ë fïd one numerical counterexample where it fails.è
è
è Let's try a = 2, b = 3, å c = 4.èèèè2 + 3·4 = (2 + 3)·(2 + 4)èè
èèèèèèèèèèèèèèèèèèèèèèè2 + 12 = 5·6
èèèèèèèèèèèèèèèèèèèèèèèèè14 = 30èè
è This contradiction implies that a + b·c = (a + b)·(a + c) is not
universally true.èèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèè
Ç Bè
5
è Please prove ê followïg statement is universally true by deductive
proç, or show that it is not universally true by counterexample.
èèèèèIf b å c are additive ïverses ç a, ên b = c.
èèèA) True by deductive proçèèèB) Not true by counterexampleè
üèè If b å c are additive ïverses ç a, ên b = c.èèèèè
Proç: Statementèèèèèèèèèèè Reason
èèè 1. a + b = 0èèèèèèèèèè1. Given additive ïverse
èèè 2. a + c = 0èèèèèèèèèè2. Given additive ïverse
èèè 3. a + b = a + cèèèèèèèè3. Transitive axiomèèèèèèèèèè
èèè 4. b + (a + b) = b + (a + c)èè4. Addition axiom for equationsèèèèèèèèèèèèèèèèèèèè
èèè 5. (b + a) + b = (b + a) + cèè5. Associative axiom for addition
èèè 6. 0 + b = 0 + cèèèèèèèè6. Additive ïverse
èèè 7. b = cèèèèèèèèèèèè7. Additive identityèèèèèèèèèèè
Ç Aè