An identity is a mathematical sentence that is always true.
For example,
[beautiful math coming... please be patient]
‘$\,x+x=2x\,$’ is an identity.
A contradiction is a mathematical sentence that is always false.
For example,
[beautiful math coming... please be patient]
‘$\,x=1\text{ and }x\ne 1\,$’ is a contradiction.
Given any mathematical statement $\,S\,$,
[beautiful math coming... please be patient]
$S\text{ or } (\text{not }S)\,$ is an identity, and
$S\text{ and }(\text{not }S)\,$ is a contradiction,
as the truth table below shows:
[beautiful math coming... please be patient] $\,S\,$  $\,\text{not }S\,$  $\,S\text{ or }(\text{not }S)\,$  $\,S\text{ and }(\text{not }S)\,$ 
T  F  T  F 
F  T  T  F 
Here's the intuition for ‘$S\text{ or } (\text{not }S)\,$’: you're either true, or you're not true.
Here's the intuition for ‘$S\text{ and }(\text{not }S)\,$’: you can't be true and not true at the same time.
The proof by contradiction technique can be applied to any type of mathematical sentence:
This approach is justified by the truth table below, where it is
shown that a statement $\,S\,$ is equivalent to: $\,(\text{not }S)\Rightarrow \text{(a contradiction)}$
[beautiful math coming... please be patient] $S$  $\text{not }S$  a contradiction  $(\text{not }S)\Rightarrow \text{(a contradiction)}$ 
T  F  F  T 
F  T  F  F 
There are three standard approaches for proving an implication, $\,A\Rightarrow B\,$:
Recall from
‘If... Then...’ Sentences
that the only time an implication is false is when the hypothesis is true, and the conclusion is false.
In a direct proof, we show that this line of the truth table can never occur.
The next method is based on the fact that an implication is equivalent to its contrapositive.
Observe that ‘proof by contraposition’ is just a direct proof of the contrapositive.
Applying ‘proof by contradiction’ to an implication leads to a proof technique that
is called an ‘indirect proof’.
By the way, don't mix up ‘proof by contraposition’ and ‘proof by contradiction’.
The words sound similar, but they're different proof techniques.
What's the only time than an implication ‘$\,A\Rightarrow B\,$’ is false?
When $\,A\,$ is true, and $\,B\,$ is false.
In an indirect proof, we assume these conditions, and then reach a contradiction.
In
Introduction to the TwoColumn Proof,
you'll get lots of practice with direct and indirect proofs, and proof by contraposition.
However, there are some skills you'll probably want to develop first—so keep moving forward!
On this exercise, you will not key in your answer. However, you can check to see if your answer is correct. 
PROBLEM TYPES:
