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當k=0的時候,y=b,假如繞原點逆時針旋轉90°,那麼該直線與x軸垂直,是沒有斜率的.由於原來的直線過(0,b)點,而新直線旋轉90°後,直線上每個點都旋轉了90°,所以(0,b)變成了(-b,0),所以新直線的方程式x=-b.至於為什麼是-b而不是b,你可以這樣理假如b>0,那麼逆時針轉90°,那麼(0,b)就到了x的負半軸上;假如b
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