quod erat demonstrandum
Concise Oxford English Dictionary © 2008 Oxford University Press:
quod erat demonstrandum /kwɒd ˌɛrat dɛmənˈstrandʊm/
used, especially at the conclusion of a formal proof, to convey that something demonstrates the truth of one's claim.
– origin L., lit. ‘which was to be demonstrated’.
'quod erat demonstrandum' also found in these Oxford entries:

