4x^2 - 20 = 0
4x^2 = 20
x^2 = 20/4 = 5
x = +-корень(5)
2x - 8x^2 = 0
x - 4x^2 = 0
x(1-4x) = 0
x1 = 0 x2 = 1/4
2x^2 - 7x + 6 = 0
D = 49 - 4*2*6 = 1
x1 = (7 + 1)/4 = 2 x2 = (7 - 1)/4 = 3/2 = 1_1/2
x^2 - x = 2x - 5
x^2 - 3x + 5 = 0
D = 9 - 4*5 < 0 нет действительных корней...
