求翻译：球极投影的研究主要介绍的是球面几何和球极投影的相关知识。首先，介绍了球极投影的定义及其坐标表达式、球面上过两点的直线（大圆）、球面上两点之间的距离和球面上两点投影到复平面后两点之间的距离；然后，介绍了球面上两条直线（大圆）所成的夹角和这两条直线投影到复平面后所成的夹角；最后，介绍了球面上三条直线（大圆）所组成的三角形、三角形的相关定义及其面积的求法、三角形投影到复平面后的面积。是什么意思？

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