Then the solution is:

∴y=4x+600…………？ 2 points

Verification: when x=200, y=4×200+600= 1400.

Conclusion: The conjecture holds, that is, y = 4x+600.............3 points.

(2)? w=y﹒p=(4x+600)﹒(-5x+9000)

x 1 = from(4x+600)(-5x+9000)= 0？ - 150,? x2？ = 1800

∴ Symmetry axis:?

∵ The opening is downward, and the symmetry axis x=825, and x is a multiple of 100.

When x=800,? =(4×800+600)(-5×800+9000)= 19000000

When the government subsidizes 800 yuan per mu, the nursery income in this area is the largest, with the maximum value of 19000000 yuan. ……? 7 points

(3) When x=800, Y = 4x+600 = 4x800+600 = 3800 (mu).

P =-5x+9000 =-5x800+9000 = 5000 (yuan)

From the meaning of the question: 3800 (30-a)% × [5000 (1+3a%)-1000] = 3990000 ..................................... 9 points

Finishing: 3a2- 10a-300=0

△ =(- 10)2-4×3×(-300)=3700

∴?

∴ a ≈11.8 =12 (omitted)

A: The value of A is about 1? 2.？ ……