\(x\left(x-5\right)^2-4x+20=x\left(x-5\right)^2-4\left(x-5\right)=\left(x-5\right)\left[x\left(x-5\right)-4\right]=\left(x-5\right)\left(x^2-5x-4\right)\)
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x(x - 5)2 - 4x + 20 = 0
x(x - 5)2 - (4x - 20) = 0
x(x - 5)2 - 4(x - 5) = 0
(x - 5)[x(x - 5) - 4] = 0
(x - 5)(x2 - 5x - 4) = 0
=> x - 5 = 0 hoặc x2 - 5x - 4 = 0
=> x = 5 x2 - 5x = 4
=> x(x - 5) = 4
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