А) x2-8x+15=0; в) x2-4x+3=0;

Б) x2-12x+20=0; г) x2+6x=8=0.

А) х² - 8 х + 15 = 0

a=1, b = - 8, c = 15

D = b² - 4ac

D = (-8) ² - 4*1*15 = 64 - 60 = 4 = 2²

D>0 - два корня уравнения

х₁, ₂ = (-b (+, -) √D) / 2a

x₁ = (8-2) / (2*1) = 6/2=3

x₂ = (8+2) / 2 = 10/2 = 5

б) х²-4 х + 3 = 0

D = (-4) ² - 4*1*3 = 16-12=4=2²

x₁ = (4-2) / 2 = 1

x₂ = (4+2) / 2 = 6/2 = 3

в) х²-12 х+20=0

D = (-12) ² - 4*1*20 = 144-80 = 64 = 8²

x₁ = (12-8) / 2 = 4/2=2

x₂ = (12+8) / 2 = 20/2=10

г) проверь знак перед 8.

x² + 6x + 8 = 0

D = 36 - 4*1*8 = 36-32=4=2²

x₁ = (-6-2) / 2 = - 8/2=-4

x₂ = (-6+2) / 2 = - 4/2 = - 2

или

х²+6 х - 8=0

D = 36 - 4*1 * (-8) = 36+32 = 68

x₁ = (-6 - √68) / 2 = (-6 - 2√17) / 2 = 2 (-3-√17) / 2 = - 3 - √17

x₂ = - 3 + √17