设圆上的点A（2，3）关于直线x+2y=0的对称点仍在圆上，且与直线x﹣y+1=0相交的弦长为2 2 ，求圆的方程．

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解：设所求圆的圆心为（a，b），半径为r，

∵点A（2，3）关于直线x+2y=0的对称点A′仍在这个圆上，

∴圆心（a，b）在直线x+2y=0上，

∴a+2b=0，①

（2﹣a）²+（3﹣b）²=r²．②

又直线x﹣y+1=0截圆所得的弦长为2 $\sqrt{}$

This summer, Mary, Tom, Alan are going to have an ideal vacation .
The followings are their plans for the coming vacation .

	Mary	Tom	Alan
Where ?	The Great Wall	Spain	Canada
When?	The first week in June	The last day of the May	June, Monday 18^th
Who ?	His parents	His best friends	His cousin
What ?	Going hiking , Going sightseeing, Visiting some famous places in Beijing, Going shopping	Going sightseeing Going shopping Going camping Watching bull(牛)-fight	Spending time in the countryside , Renting videos at night , Going fishing , Going hiking, Sleeping a lot
How long?	About one month	Only two weeks	Until September
Transportation	By plane and by bus	By boat and by plane	By plane and by train