7)2x^2 + 7x - 4 = 0
2x^2 - x + 8x - 4 = 0
(2x^2 + 8x) - (x + 4) = 0
2x.(x + 4) - (x + 4)= 0
(x + 4).(2x - 1) = 0
TH1: x + 4 = 0
x = 0 - 4
x = -4
TH2: 2x - 1 = 0
2x = 0 + 1
2x = 1
x = 1 : 2
x = 1/2.
=> x thuộc {-4 ; 1/2}.
8)(2x + 1).(4x^2 - 2x + 1) - 8x.(x^2 + 2) = 17
8x^3 + 1 - 8x^3 - 16x = 17
(8x^3 - 8x^3) + 1 - 16x = 17
1 - 16x = 17
16x = 1 - 17
16x = -16
x = -16 : 16
x = -1
9) x^3 - 7x - 6 = 0
x^3 - x - 6x - 6 = 0
x.(x^2 - 1) - 6.(x + 1) = 0
x.(x - 1).(x + 1) - 6.(x + 1) = 0
(x + 1).[x.(x - 1) - 6] = 0
(x + 1).(x^2 - x - 6) = 0
(x + 1).(x^2 - 3x + 2x - 6) = 0
(x + 1).[(x^2 + 2x) - (3x + 6)] = 0
(x + 1).[x.(x + 2) - 3.(x + 2)] = 0
(x + 1).(x + 2).(x - 3) = 0
TH1: x + 1 = 0
x = -1
TH2: x + 2 = 0
x = -2
TH3: x - 3 = 0
x = 3
=> x thuộc {-1 ; -2 ; 3}
10)x^3 + 27 + (x + 3).(x - 9) = 0
x^3 + 3^3 + (x + 3).(x - 9) = 0
(x + 3).(x^2 - 3x + 9) + (x + 3).(x - 9) = 0
(x + 3).(x^2 - 3x + 9 + x - 9) = 0
(x + 3).(x^2 - 2x) = 0
TH1: x + 3 = 0
x = -3
TH2: x^2 - 2x = 0
x.(x - 2) = 0
THa: x = 0
THb: x - 2 = 0
x = 2.
=> x thuộc {-3 ; 0 ; 2}
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