3.
(x - 1)(x + 2) = ax^2 - bx - 5
x^2 + x - 2 = ax^2 - bx - 5
=> a = 5/2, b = -5/2
x = -1 代入 x^2 + ax + 3 = 0 可得 a = 4
(x - 4)(x - b) = x^2 + 5x + c = 0
x^2 + (-4-b)x + 4b = x^2 + 5x + c = 0
=> -4-b = 5, b = -9
=> c = 4b = -36
(1)
3a + 2b + c = 12 - 18 - 36 = -42
(2)
ax^2 + 2bx + c = 0
4x^2 - 18x - 36 = 0
(x - 6)(2x + 3) = 0
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