In mathematics, one of the most famous ways of proving theorems and propositions is to prove by contradiction. Actually, this is a logical way of thinking based on "the law on non-contradiction (LNC)" as first formalized as a metaphysical principle by Aristotle.
This way of proving says that a statement P and its negation (non)P can not happen at the same time. For example, day and night can not happen at the same time; similarly, left and right, up and down, and full and empty.
So, to prove a theorem or a proposition, you may either prove that its statement is right, or the negation of its statement is wrong.
Using the contradiction in proving a theorem or a proposition in mathematics starts usually by assuming that the statement of this theorem or proposition is wrong. Then, the researcher should find a mistake as a result of his assumption. Thus, the statement must not be wrong by contradiction, and so it is true.
For example, we will prove, using contradiction, the following statement P: "If x^2 is even, then x is even". Suppose to get a contradiction that x is odd, that is x=n+1, where n is an even number. So, x^2=(n+1)^2=n^2+2n+1. Then x^2-(n)^2-2n=1 an odd number, hence the number y= x^2-(n)^2-2n is an odd number. On the other hand, x^2 is even, (n)^2 is even since n is even and 2n is even, which implies that y is even. So, y is odd and even at the same time, which is a contradiction. Therefore, the statement P is true.
Conclusion
Although there are a lot of ways for proving theorems and propositions in mathematics, proving using contradiction stays the more famous and familiar way mathematicians like to use.