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FERDINAND D. AGDORO JR.
PROOF BY
CONTRADICTION
2+2 =?
2+2 =45
1
2+2 =
2+2 =
CONTRADICTI
NG
deny
rebut
oppose
PROOF BY
CONTRADICTION
*PROOF - proof is a rhetorical device for convincing
someone else that a mathematical statement is true or
valid.
*CONTRADICTION - a proposition, statement, or
phrase that asserts or implies both the truth and falsity
of something
PROOF BY CONTRADICTION -- a form of proof,
and more specifically a form of indirect proof, that
establishes the truth or validity of a proposition by
showing that the proposition's being false.
PROOF BY
CONTRADICTION
 This is not limited to proving just conditional
statements – it can be used to prove any kind of
statement whatsoever.
The basic idea is to assume that the statement
we want to prove is false, then show that this
assumption leads to nonsense. We are then led
to conclude that we were wrong to assume the
statement was false, so the statement must be
true.
TRU
E
STATEMEN
T
FALS
E
nonsens
e
TRU
E
 (also known as reductio ad absurdum, Latin for
"by reduction to the absurd"), it is shown that if
some statement were true, a logical contradiction
occurs, hence the statement must be false.
PROOF BY
CONTRADICTION
A real number x is RATIONAL if x = a/b for some a, b are integers. Also, x is
IRRATIONAL if it is not rational, that is if x ≠ a/b for every a, b are
integers.
 DEFINITION
STEPS
EXAMPLE PROVE BY CONTRADICTION
is an irrational number
is an irrational number
 PROPOSITIO
N
 PROOF
 PROOF
EXAMPLE PROVE BY CONTRADICTION
Suppose a is an integer. If a2 is
even, then a is even.
Suppose n is an integer.
If n is odd, then n2 is odd.
QUESTIONS
THANK YOU !

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Proof by contradiction

  • 1. FERDINAND D. AGDORO JR. PROOF BY CONTRADICTION
  • 5. PROOF BY CONTRADICTION *PROOF - proof is a rhetorical device for convincing someone else that a mathematical statement is true or valid. *CONTRADICTION - a proposition, statement, or phrase that asserts or implies both the truth and falsity of something PROOF BY CONTRADICTION -- a form of proof, and more specifically a form of indirect proof, that establishes the truth or validity of a proposition by showing that the proposition's being false.
  • 6. PROOF BY CONTRADICTION  This is not limited to proving just conditional statements – it can be used to prove any kind of statement whatsoever. The basic idea is to assume that the statement we want to prove is false, then show that this assumption leads to nonsense. We are then led to conclude that we were wrong to assume the statement was false, so the statement must be true.
  • 8.  (also known as reductio ad absurdum, Latin for "by reduction to the absurd"), it is shown that if some statement were true, a logical contradiction occurs, hence the statement must be false. PROOF BY CONTRADICTION
  • 9. A real number x is RATIONAL if x = a/b for some a, b are integers. Also, x is IRRATIONAL if it is not rational, that is if x ≠ a/b for every a, b are integers.  DEFINITION STEPS EXAMPLE PROVE BY CONTRADICTION is an irrational number is an irrational number  PROPOSITIO N  PROOF
  • 11. EXAMPLE PROVE BY CONTRADICTION Suppose a is an integer. If a2 is even, then a is even.
  • 12. Suppose n is an integer. If n is odd, then n2 is odd.
  • 14.
  • 15.