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An indirect proof is a proof wherein you begin by assuming temporarily that the desired conclusion is not true which then by reasoning logically reaches to a certain contradiction or some other known fact.

1. Assume temporarily that the conclusion is not true.

2. Reason logically until you reach a contradiction.

3. Point out that the assumption was wrong and the conclusion must then be true.

Consider the following statement:If $\stackrel{炉}{EF}鈮�\stackrel{炉}{GH},$then $\stackrel{鈫�}{EF}$and $\stackrel{鈫�}{GH}$aren鈥檛 parallel.

In order to write out the first sentence of an indirect proof of this statement, state the temporary assumption that the given conclusion is not true.

Therefore, in this case, the first sentence to be used to write this proof is: 鈥淎ssume that $\stackrel{炉}{EF}鈮�\stackrel{炉}{GH}$鈥�.