pure maths

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2011-12-07 12:00:17 補充：
感謝Rvy的指正

因為我無入個direct link 度睇,所以唔見有'xy>0', 我以為個題目suppose P^(-1) 存在。

補充:

∵xy>0
∴(x>0 and y>0) or (x<0 and y<0)

when x>0 and y>0
det(P) = - x - y < 0

when x<0 and y<0
det(P) = - x - y >0

∵det(P) is not equal to 0
∴P^(-1) exists

小王 ans miss the checking part

|P|=-y-x=-(x+y)
As xy>0
(x<0 and y<0) or (x>0 and y>0)
i.e. x+y=/=0
|P|=/=0
P-1 exists.

then do the task followed by 小王's ans.