[tex](2x + 1) {}^{2} - (2x - y)(2x + y) = (y + 8)(y - 10) \\ 4x(x - 5) - (2x - 3)(2x - 9) = 6y - 104 \\ \\ {4x}^{2} + 4x + 1 - 4 {x}^{2} + y {}^{2} = {y}^{2} - 10y + 8y - 80 \\ {4x}^{2} - 20x -4 {x}^{2} + 18x + 6x - 27 = 6y - 104 \\ \\ 4x + {y}^{2} - {y }^{2} + 2y = - 80 - 1 \\ 4x - 6y = - 104 + 27 \\ \\ 4x + 2y = - 81 \\ 4x - 6y = - 77 \: \: \: | \times ( - 1) \\ \\ 4x + 2y = - 81 \\ - 4x + 6y = 77 \\ \\ 4x - 4x + 2y + 6y = 77 - 81 \\ 8y = - 4 \\ y = - 4 \div 8 \\ y = - 0.5 \\ \\ 4x + 2y = - 81 \\ 4x + 2 \times ( - 0.5) = - 81 \\ 4x - 1 = - 81 \\ 4x = - 81 + 1 \\ 4x = - 80 \\ x = - 80 \div 4 \\ x = - 20 \\ x + y = - 20 - 0.5 = - 20.5[/tex]

Ответ: - 20,5

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