如果一个四边形既有内切圆又有外接圆,我们把这样的四边形称为双心四边形.双心四边形既有外心又有内心.根据双心四边形的定义,它必须满足两个条件,第一,四条边的垂直平分线相交于一点,即外心;第二,四个角的平分线相交于一点,即内心.显然,正方形是双心四边形,它的外心与内心重合为一点. If a quadrilateral has both inscribed and circumscribed circles, we refer to such a quadrilateral as a bicentric quadrilateral. A bicentric quadrilateral has both an outer heart and an inner heart. According to the definition of a bicentric quadrilateral, it must satisfy two conditions. The vertical bisectors of one side and four sides intersect at one point, namely the outer heart; the second, the bisectors of the four corners intersect at one point, namely the inner heart. Obviously, the square is a bicentric quadrilateral whose outer heart coincides with the inner heart.