Solve Proving Proofs

Mathematical statement considered the sequence of statements by proofs, every statement being adjusted with some definition or a proposition or an axiom that is before launched by the method of deduction using only the permitted logical rules. Repeatedly we prove a proposition straightly from what is given in the proposition. But so many times it is easier to prove a same proposition not proving the proposition by itself,

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Solve Proving Proofs - Types of Proofs:

Solve Proving Proofs Indirect Method:

Solve proving proofs - Indirect method:

Prove the given function f : R  =>  R defined by f(x)  =  2x  +  5 is one – one.

Sol :  A function is one – one if  f(x1)  =  f(x2)  => x1  =  x2.

Using this we have to show that “ 2x1 + 5  = 2x2  +  5” => “  x1  =  x2”.This is of the form p  =>  q, where , p is 2x1  +  5 =  2x2  +  5 and q : x1  =  x2. so this is the direct method proofing.

We can also prove the same by using contrapositive of the statement. Now contrapositive of this statement is ~ q=>  ~ p, is, contrapositive of “if f(x1)  =  f(x2), then  x1  =  x2” is “if x1  =  x2, then f(x1) =  f(x2)”.

Now                 x1  !=   x2

=>                   2x1  !=   2x2

=>                   2x1  +  5  !=   2x2  + 5

=>                   f(x1)  !=   f(x2).
Solve Proving Proofs of Direct Approach:

Solve Proving Proofs -direct Proof:


It  is the proof of  a proposition in which we directly start the proof  with what is given in the proposition.

1)Straight forward approach.

2) Mathematical Induction approach.

3)Proofs By cases or by   exhaustion.

Solve Proving Proofs -Indirect Proof:

Instead of proving the proposition directly, we establish the proof of the proposition through  proving a proposition which is equivalent to the given proposition.

1)Proofs  by contradiction.

2)Proofs by using contrapositive statement.

3)Proofs by a by a counter example.


My forthcoming post is on free math word problems for 2nd grade and ntse 2013 syllabus will give you more understanding about Algebra.


Solving Proving Proofs - some Examples for Proving Proofs:

Solving proving proofs - Direct Method:

Prove the given function f:  R  =>  R

Defined by f(x) = 2x + 5 is one – one.

Sol :  Note that a function f is one – one if

f(x1)  =  f(x2)    x1  =  x2 (definition of one – one function)

Now given that   f(x1)  =  f(x2),

=> 2 x1 +  5 = 2 x2 +  5

=>                 2 x1  +  5  –  5  = 2 x2  +  5  –  5(adding the same quantity on both sides)

=>                 2 x1  +  0  =  2 x2  +  0

=>                 2 x1  =  2 x2 (using additive identity of real number)

=>                 (2)/(2) x1  =  (2)/(2) x2 (dividing by the same non zero quantity)

=>                 x1  =  x2.