Rationalize v. t. : To make rational; also, to convert to rationalism.

Rationalize v. t. : To interpret in the manner of a rationalist.

Rationalize v. t. : To form a rational conception of.

Rationalize v. t. : To render rational; to free from radical signs or quantities.

Rationalize v. i. : To use, and rely on, reason in forming a theory, belief, etc., especially in matters of religion: to accord with the principles of rationalism.

Example:
How do we Rationalize the atomic bombings of Hiroshima and Nagasaki?

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Defination from the web:
rationalize from the Merriam-Webster Online Dictionary with audio pronunciations, thesaurus, Word of the Day, and word games. ... 2 : to free (a mathematical expression) from irrational parts <rationalize a denominator> ...