建立以C为原点,DC为X轴的平面直角坐标系
则向量AD=(0,1) AB=(2,0)圆C的方程:x²+y²=R²
∵DC∥AB,所以∠CDB=∠ABD,所以直角△ADB∽直角△QCD(Q为圆与BD的切点)
所以QC/AD=CD/BD ∴QC=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408737371.png)
=R
设P(x,y) 因为P在圆上或园内,∴其坐标满足:x²+y²≤
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408753308.png)
向量
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408924391.png)
=(x+1,y+1)=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408940506.png)
+
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408971480.png)
=(
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408987508.png)
)
从而:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409002392.png)
="x+1,"
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409034323.png)
=y+1 ∴ (
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409002392.png)
-1)²+(
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409034323.png)
-1)²≤
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408753308.png)
可以推断,当P在圆上时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408612415.png)
达到最大值, 此时:(
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409002392.png)
-1)²+(
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409034323.png)
-1)²=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408753308.png)
设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409002392.png)
-1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408737371.png)
cosA,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409034323.png)
-1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408737371.png)
sinA 所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408612415.png)
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409330441.png)
(cosA+2sinA)+
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409346388.png)
由于cosA+2sinA=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409361322.png)
sin(A+B) 所以最大值取
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409361322.png)
,所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211408612415.png)
的最大值为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409330441.png)
X
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409361322.png)
+
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823211409346388.png)
=2
赞同
最小值为1.