在 中, 設點 在 邊上且 平分 , 並設 的中點為 。設以 為直徑的圓 與 交於點 , 以 為直徑的圓 與 交於點 。證明 四點共圓。
Let be the midpoint of the internal bisector of . Circle with diameter intersects at and circle with diameter intersects at . Show that belong to the same circle.
在 中, 設點 在 邊上且 平分 , 並設 的中點為 。設以 為直徑的圓 與 交於點 , 以 為直徑的圓 與 交於點 。證明 四點共圓。
Let be the midpoint of the internal bisector of . Circle with diameter intersects at and circle with diameter intersects at . Show that belong to the same circle.
If , then the statement is obvious. Without loss of generality, we assume that . Let be the common chord of the given circles, as shown in the figure. We draw the line which passes through and is perpendicular to and denote by and the points of intersection of the given lines with and respectively.
We prove that passes through . Let be the point of intersection of and . Since we have the following proportions:
Thus, , . Also, and triangles and are similar. Thus, , . Hence, and coincides with . By analogy, we can show that passes through too. We have
This implies that belong to the same circle. is inscribed in the circle and thus
which implies that lie on the circle.
We have shown that lie on the same circle. In the right angled triangle , is a median. Therefore, . Thus
Consider triangles and that have the common angle and
We have that are concyclic, which implies the result.