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Proof by contradiction normally follows this strategy.
To prove Statement P is true:
First assume the inverse of statement P to be true and show that this leads to two conflicting statements
(a) Statement R is true and (b) Statement R is false.
This is a contradiction, therefore the original statement P must be true.
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!! Discussion
One proof of the Infinity of Prime Numbers uses proof by contradiction.
Assume that there exist a finite number of primes EQN:p_1,p_2,p_3,...,p_n
That is to say that every number greater than EQN:P_n is a multiple of at least one of the numbers EQN:p_1,p_2,p_3,...,p_n
Consider the number EQN:N=p_1.p_2.p_3...p_n + 1
Now N > EQN:P_n and N is not divisible by any of the primes EQN:p_1,p_2,p_3,...,p_n as the remainder is always 1.
This is a contradiction, therefore the initial assumption that there exists a finite number of primes must be wrong.
Please note: This does not mean that N is prime as there may exist a prime between EQN:P_n and N which divides N.