课本里有这样一个定理:在直角三角形中,30°所对的直角边等于斜边的一半．在习题里给出了该定理的逆定理:在直角三角形中, 如果一条直角边等于斜角的一半,那么这条直角边所对的锐角是30°．逆定理的证法可仿照定理的证法,很容易完成(用加倍法或截半法)．该定理还有另外一个逆命题:“在一个三角形中,如果30°所对的边等于另一边的一半, 那么这个三角形是直角三角形．”这个命题是 There is a theorem in the textbook: In a right-angled triangle, the right side of 30° is equal to half of the hypotenuse. The inverse theorem of the theorem is given in the exercises: In a right-angled triangle, if a right-angled edge is equal to half the bevel, then the acute angle of this right-angled edge is 30°. The proof of the opposite theorem can be modeled on the proof of the Theorem, and it is easy to complete (by doubling or halving). The theorem has another converse: “In a triangle, if the 30° edge is equal to half of the other side, then the triangle is a right triangle.” This proposition is