Word 1: Rational (RASH uh nuhl)
Meaning 1: Having reason or understanding.
Example Sentence: He was too confused to make a rational decision.

Meaning 2: Thinking about what you do in a calm way, and making sensible and intelligent decisions.
Example Sentence: The problem can only be sorted if discussed in a rational way.

Meaning 3: Having the ability to explain in a sensible manner.
Example Sentence: The rational for the hasty exit could not be guessed.

Word 2: Rationale (rash uh NAL)
Meaning: An underlying reason.
Example Sentence: The chairperson had to explain the rationale behind the decisions he had taken.

Example Sentence using both words:
To take a rational decision he had to study the rationale of the policy.