Proof: AC = df.

25. Simplify before evaluating.

3-2xy+2yx2+6xy-4x2y, where x =- 1 and y =-2.

Four, answer questions (***35 points)

26.(8 points) It is known that the straight line Y = 2x+ 1.

(1) Find: the coordinate of the intersection point A of the straight line and the Y axis is known;

(2) if the straight line l: y = kx+b is symmetrical with the known straight line about y, find the values of k and b.

27.(8 points) It is known that the straight line y=kx+b passes through the point P (0,2) on the Y axis, and the graphic area enclosed by the two coordinate axes is 1. Find the resolution function of a straight line.

28.(9 points) It is known that in the triangle AB=AC, ∠ A = 90, AB = AC, and D is the midpoint of BC.

(1) As shown in the figure, e and f are points on AB and AC respectively, and be = af.

It is proved that △DEF is an isosceles right triangle.

(2) If E and F are points on the extension lines of AB and CA respectively, there is still Be = AF, and other conditions remain unchanged.

So, is △DEF still an isosceles right triangle? Prove your conclusion.

29.( 10) As shown in the figure, the side length of the square OABC is 2, O is the origin of the rectangular coordinate system, and points A and C are on the X axis and Y axis respectively. Point P moves along the side of the square in the order of O→A→B, the distance of point P is X, and the area of △ OPB is Y. 。

(1) Find the functional relationship between Y and X, and write the range of the independent variable X;

(2) exploration: when, the coordinates of point P;

(3) Is there a straight line passing through the point (0,-1) that bisects the square OABC area? If it exists, find the analytical formula of this straight line; If it does not exist, please explain why.