设OP=(x,y)
AB=OB-OA=(cosa-sina,-1)
BP=(x-cosa,y)
AB=BP
cosa-sina=x-cosa,x=2cosa-sina
y=-1
OP=(2cosa-sina,-1)
PB=(sina-cosa,1)
CA=(2sina,-1)
f(a)=PB*CA
设OP=(x,y)
AB=OB-OA=(cosa-sina,-1)
BP=(x-cosa,y)
AB=BP
cosa-sina=x-cosa,x=2cosa-sina
y=-1
OP=(2cosa-sina,-1)
PB=(sina-cosa,1)
CA=(2sina,-1)
f(a)=PB*CA