Let ABC be a circle, let the angle BEC be an angle at its centre, and the angle BAC an angle at the circumference, and let them have the same circumference BC as base;
I say that the angle BEC is double of the angle BAC.
IN circulo ABC, cuius centrum D, superbasin BC, consticuatur angulus BDC, ad centrum; et super eandem basin angulus BAC, ad peripheriam.
Dico angulum BDC, duplum esse anguli BAC.
解曰。甲乙丙圜。其心丁。有乙丁丙分圜角。乙甲丙負圜角。同以乙丙圜分為底。
題言乙丁丙角。倍大於乙甲丙角。