rational vs unbelievable

rational

noun
  • A rational number: a number that can be expressed as the quotient of two integers. 

  • The breastplate worn by Israelite high priests. 

adj
  • Healthy or balanced intellectually; exhibiting reasonableness. 

  • Logically sound; not self-contradictory or otherwise absurd. 

  • Of an algebraic expression, capable of being expressed as the ratio of two polynomials. 

  • Expressing the type, structure, relations, and reactions of a compound; graphic; said of formulae. 

  • Of a number, capable of being expressed as the ratio of two integers. 

  • Expressing a physical object. 

  • Capable of reasoning. 

unbelievable

adj
  • Not to be believed. 

  • Amazing, incredible; so surprising it is almost unable to believe. 

  • Implausible or improbable. 

How often have the words rational and unbelievable occurred in a corpus of books? (source: Google Ngram Viewer )