Q.E.D.

which was to be shown or demonstrated (used esp. in mathematical proofs).

[1810-20; < L quod erat demonstrandum]

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Q.E.D. or QED¹ (no periods),

which was to be proved (Latin, quod erat demonstrandum): »

The layman…may find it possible to believe that Euclid—or possibly Newton—wrote a concluding QED, to indicate the end of a complete collection of all mathematics (New Scientist).

QED² (no periods),

quantum electrodynamics.

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QED [QED] (BrE) (also Q.E.D. US, BrE) [ˌkjuː iː ˈdiː] [ˌkjuː iː ˈdiː] abbreviation

that is what I wanted to prove and I have proved it (from Latin ‘quod erat demonstrandum ’)