A、B、C是任意集合,如果A∈B及B<imgdata-url="doc/a@BE62978D1556D978DD8ADCBE797A3385.jpg"data-preview_url_ueditor="doc/a@BE62978D1556D978DD8ADCBE797A3385.jpg"src="http://121.251.253.213/doc/a@BE62978D1556D978DD8ADCBE797A3385.jpg/download?time=636160156065406061&token=88BA0B3DF5103397D077146AA5FE1ABD&filename=jpg"/>C,则A∈C。