prove to be

prove (to be) (something)

1. To show or provide evidence of having a particular trait, attribute, or characteristic. The new method proved to be useful in detecting radiation. She's proven a reliable ally in my time at this company.
2. To show or provide evidence that someone, something, or oneself has a particular trait, attribute, or characteristic. In this usage, a noun or pronoun is used directly after "prove." The new computer's incredibly reliable CPU and sleek design prove it to be a worthy investment for any student or professional. The week I had to spend in New York proved Janet to be more than capable of running the office on her own. This new evidence proves him a liar.
See also: prove
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

prove to be something

to be shown to be someone or something; to be found to be someone or something. Susan proved to be a good friend when she lent me some money. The food proved to be spoiled when I smelled it.
See also: prove, to
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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References in classic literature ?
He assured Emily that he had only decided on writing to her, after some hesitation, in the hope of serving interests which he did not himself understand, but which might prove to be interests worthy of consideration, nevertheless.