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10. 当一条直线和另一条直线交成的邻角彼此相等时,这些角的每一个叫做直角,而且称其中一条直线垂直于另一条直线。
还有一个定义是必须提到的这就是第23个定义也就是最后一个定义是于关平行线的
23. 平行直线是在同一平面内的直线,向两个方向无限延长,在不论哪个方向它们都不相交。