數學問題！！help me!!

In geometry, the nine-point circle is a circle that can be constructed for any given triangle. It is so named because it passes through nine significant points, six lying on the triangle itself (unless the triangle is obtuse). They include:
The midpoint of each side of the triangleThe foot of each altitudeThe midpoint of the segment of each altitude from its vertex to the orthocenter (where the three altitudes meet)The nine-point circle is also known as Feuerbach's circle, Euler's circle, Terquem's circle, the six-points circle, the twelve-points circle, the n-point circle, the medioscribed circle, the mid circle or the circum-midcircle.

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/7/71/Nine-point_circle.svg/364px-Nine-point_circle.svg.png

在三角形中，三邊中點，三條高的垂足，垂心到各頂點線段的三個中點，凡九點共圓----這就是著名的九點圓定理。