Given that the left focus of hyperbola X-Y = 1 is f, point P is on the hyperbola, and the ordinate of point P is less than 0, then the value range of the slope of the straight line pf?

When the point P moves infinitely to the lower right of the hyperbola, the line PF is gradually parallel to the asymptote, but never parallel, so the inclination angle is greater than 45 degrees;
When point P approaches the vertex, the inclination angle increases gradually, but it is less than 180 degrees
So the inclination angle of the straight line PF is (45 ° 180 °)
From this, we can see the change range of the slope of the straight line PF (- ∞, 0) ∪ (1, + ∞)

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