常見例句雙語例句The paper is concerned with two aspects:A necessary and sufficient condition for convex hull of a set in E2 being closed;本論文以凸躰爲研究對象，主要涉及兩個方麪的內(nèi)容：平麪凸集的凸包爲閉集的充分必要條件； 返回 convex closed hull