20 1 1 penultimate question of the national junior high school mathematics competition

13. solution: (1) when X =- 1 and x=3, the value of y is the same, that is, Y = A-B+C = 9A+3B+C. ∴ B =-2A, then

XM =-。 ∵ point m is on Y = 3x-7, ∴ YM = 3-7 =-4, ∴ M (1, -4).

Let y = a (x- 1) 2-4, ∫ When x= 4, y = 3× 4-7 = 5. When x = 4, y = 5, 5 = a (4- 1) 2-4, a =

(2) When x= 0 and y =-3, ∴ c (0, -3). When y = 0, x 1 =- 1, x2 = 3. ∴ A (- 1，0)，B(3，

∴ S = S△AOC+S ladder ocpq =×/kloc-0 /× 3 +× (3+│ 2t-6 │ )× T.

=+t =-T2+t+.................................................................... 8 points.

(3) P 1 (2, -2), P2 (,-), P3 () ...................12.

(4) (- 1, -3) or (,-), (-) ................................15.