Methods of proof
Mathematical statements can typically be phrased as an implication, may be complex statements themselves that involve conjunctions (and), disjunctions (or), negations, quantifiers, even implications. There are various ways in which an implication can be proven true, and there is no hard and fast rule that dictates which proof method to use given a particular problem.
Direct proof
A direct proof is one of the most familiar forms of proof. We use it to prove statements of the form ”if
then
” or ”
implies
” which we can write as
. The method of the proof is to takes an original statement
, which we assume to be true, and use it to show directly that another
statement
is true. So a direct proof has the following steps:
Assume the statement
is true.
Use what we know about
and other facts as necessary to deduce that another statement
is true, that is show
is true.
Example :
Let
show that,
.
Proof by contrapositive
Proof by contrapositive, or proof by contraposition, is a rule of inference used in proofs, where one infers a conditional statement from its contrapositive. In other words, the conclusion "if
, then
" is inferred by constructing a proof of the claim "if not
, then not
" instead. More often than not, this approach is preferred if the contrapositive is easier to prove than the original conditional statement itself. We show
insted of
.
Example :
let
show that
is not divisible by
is even.
Proof by contraduction
Proof by contradiction (also known as indirect proof) is a common proof technique that is based on a very simple principle: something that leads to a contradiction can not be true, and if so, the opposite must be true. To show that
is true, we suppose that
is false and than
is true. We show that
, when
is true. So
is true. As
is true then
is true.
Example :
Show that
does not admit an inverse in
.
By induction
A proof by induction is just like an ordinary proof in which every step must be justified. Let
be a logical statement for each
. The principe of mathematical induction states that
is true all
if
is true, and
for all
.
Example :
Show that for all
, we have
.
By giving a counter example
A proof by counter example is not technically a proof. It is merely a way of showing that a given statement cannot possibly be correct by showing an instance that contradicts a universal statement. To show that a proposition
is false, we must show that
is true.
Example :
Show that the proposition:
is false.
with
,
is false.
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