а) x² + x = 90

x² + x - 90 = 0

D = 1 + 4*90 = 361 = 19²

x₁₂ = (-1 ± 19) / 2

x₁  = 9

x₂ = -10

Ответ: 9; -10

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б)-4x=7x²

7x² + 4x = 0

x(7x+4) = 0

x=0       или    7x + 4=0

                     x = -4/7

Ответ: 0; -4/7

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в) 1/5 · x² + x - 10 = 0       | · 5

x² + 5x - 50 = 0

D = 25 + 4*50 = 225 = 15²

x₁₂ = (-5 ± 15) / 2

x₁  = 5

x₂ = -10

Ответ: 5; -10

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г) x² - x = 110

x² - x - 110 =0

D = 1 + 4*110 = 441= 21²

x₁₂ = (1 ± 21) / 2

x₁  = 11

x₂ = -10

Ответ: 11; -10

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д)-3x²=11x

3x² + 11x = 0

x(3x+11) = 0

x=0       или    3x + 11=0

                     x = -11/3

Ответ: 0; -11/3

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е) 1/4 · x² - x - 3 = 0       | · 4

x² - 4x - 12 = 0

D = 16 + 4*12 = 64= 8²

x₁₂ = (4 ± 8) / 2 = 2 ± 4

x₁  = 6

x₂ = -2

Ответ: 6; -2