b) (2x + 2)^2 - 4 . (x - 1) . (x + 1) = 49 

=> (2x)^2 + 2 . 2x . 2 + 2^2 - 4 . (x^2 - 1) = 49 

=> 4x^2 + 8x + 4 - 4x^2 + 4 = 49 

=> (4x^2 - 4x^2) + 8x = 49 - 4 - 4 

=> 8x = 41 

=> x = 41/8 

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c) (2x + 1) . (1 - 2x) + (1 - 2x)^2 = 18 

=> 1 - (2x)^2 + 1^2 - 2 . 1 . 2x + (2x)^2 = 18 

=> 1 - 4x^2 + 1 - 4x + 4x^2 = 18 

=> (-4x^2 + 4x^2) - 4x = 18 - 1 - 1 

=> -4x = 16 

=> x = -4 

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d) 2 . (x + 1)^2 - (x - 3) . (x + 3) - (x - 4)^2 = 0 

=> 2 . (x^2 + 2x + 1) - (x^2 - 3^2) - (x^2 - 8x + 4^2) = 0 

=> 2x^2 + 4x + 2 - x^2 + 9 - x^2 - 8x - 16 = 0 

=> (2x^2 - x^2 - x^2) + (4x - 8x) = 0 - 2 - 9 + 16 

=> -4x = 5 

=> x = -5/4 

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e) (x - 5)^2 - x . (x - 4) = 9 

=> x^2 - 2 . x . 5 + 5^2 - x^2 + 4x = 9 

=> x^2 - 10x + 25 - x^2 + 4x = 9 

=> (x^2 - x^2) + (-10x + 4x) = 9 - 25 

=> -6x = -16 

=> x = 16/6 = 8/3 

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f) (x - 5)^2 + (x - 4) . (1 - x) = 0 

=> x^2 - 10x + 25 + x . (1 - x) - 4 . (1 - x) = 0 

=> x^2 - 10x + 25 + x - x^2 - 4 + 4x = 0 

=> (x^2 - x^2) + (-10x + x + 4x) = 0 - 25 + 4 

=> -5x = -21 

=> x = 21/5 

#tueluongtai