จากเวปไซต์นี้ค่ะ
http://jwilson.coe.uga.edu/EMAT6680F...s/Squares.html
Proof:
1) To show EFGH and IJKL are squares, I used GSP. As you can see below picture, first I draw midpoints of sides AB, BC, CD, and DA then when I connected opposite midpoints H,L,J, and F lies on the segment MN and E,I,K, and G lies on the segment OP. Thus all vertices lie on whether the segment MN or OP. Since ABCD is a square MN and OP are perpendicular to the sides of ABCD. Also we know the segments AH, AG, BE, BH, CF, CE, DG, and DF trisect the angles A,B,C, and D. Since MN is perpendicular to OP then they make a 90 degree angle in their intersection. Therefore, the distances from that intersection point to the vertices of EFGH and IJKL are the same. This means the vertices of these two quadrilaterals have angles 90 degree. Thus, EFGH and IJKL are squares.
We can also see this solution from below picture. Since the vertices H,L,J, F and E,I,K,G lie on the perpendicular segments MN and OP, EFGH and IJKL are congruent to the ABCD square therefore these two are squares.
ช่วยแปลให้หน่อยค่ะ...พอดีแปลแล้วยังไม่ค่อยเข้าใจค่ะ...ขอบคุณค่ะ