设BC＝a,m＝a sinθ×a conθ/2＝a²x/4
设DE＝b a＝b cotθ+b+b tanθ＝b﹙x+2﹚/x n＝b²＝a²x²/﹙x+2﹚²
∴y＝m/n＝﹙x+2﹚²/﹙4x﹚＝x/4+1+1/x