`#3107.101107`
a)
`(3 + x)^2 - (x - 1)(x + 1) = 1`
`<=> 9 + 6x + x^2 - (x^2 - 1) = 1`
`<=> 9 + 6x + x^2 - x^2 + 1 = 1`
`<=> 6x + 10 = 1`
`<=> 6x = 1 - 10`
`<=> 6x = -9`
`<=> x = -3/2`
Vậy, `x = -3/2`
b)
`(9 + x)(9 - x) + x(x - 7) = 11`
`<=> 81 - x^2 + x^2 - 7x = 11`
`<=> 81 - 7x = 11`
`<=> 7x = 70`
`<=> x = 10`
Vậy, `x = 10`
c)
`(2x - 3)(2x + 3) - 4(x - 1)^2 = 19`
`<=> 4x^2 - 9 - 4(x^2 - 2x + 1) = 19`
`<=> 4x^2 - 9 - 4x^2 + 8x - 4 = 19`
`<=> 8x - 13 = 19`
`<=> 8x = 32`
`<=> x = 4`
Vậy, `x = 4.`
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Sử dụng các hđt:
1. `A^2 - B^2 = (A - B)(A + B)`
2. `(A - B)^2 = A^2 - 2AB + B^2`
3. `(A + B)^2 = A^2 + 2AB + B^2`
d)
Đúng 3
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