举报
江岚蓝
以DC,DA为x,y轴建立直角坐标系,设C(1,0),N(0,n),M(x,y),则B(1,1), 等腰直角△CMN以CN为斜边(改题了), ∴MC=MN,MC^2+MN^2=CN^2, ∴(x-1)^2+y^2=x^2+(y-n)^2,① (x-1)^2+y^2+x^2+(y-n)^2=1+n^2,② 由①,2ny+1-n^2=2x, x=ny+(1-n^2)/2,③ 由②,x^2-x+y^2-ny=0,④ 把③代入④,[ny+(1-n^2)/2]ny+(-1-n^2)/2]+y^2-ny=0, (ny-n^2/2)^2-1/4+y^2-ny=0, (n^2+1)y^2-(n^3+n)y+n^4/4-1/4=0, △=(n^3+n)^2-(n^2+1)(n^4-1)=n^4+2n^2+1=(n^2+1)^2, y1=(n+1)/2,y2=(n-1)/2, 代入③,x1=(n+1)/2,x2=(1-n)/2. 当x=(n+1)/2时BE=(1-n)/2,2BE=1-n, 如图1,2,n<0,CD+DN=1-n=2BE. 如图3,n>0,CD+DN=1+n>2BE,宜改为CD-DN=1-n=2BE. 当x=(1-n)/2时BE=(1+n)/2,2BE=1+n,留给您练习.