1)5х^3-20х^2=0 2)9x-x^3=0 3)x^3+3x^2-4x-12=0 4)x^3-2x^2=0 5)9x^2-0.25=0 6)64-9x^2=0 7)2x^3-x^2-18x+9=0 8)x^2-6x+9=0 9)x^2+10x+25=0 10)4x^2-4x+1=0 11)9x^2+6x+1=0 12)3x-0.4x^2=0 13)-0.4x^2+8x=0

1) 

5x² = 0 или x-4=0
x = 0 или x=4
2) 9x - x³ = 0
x(9 - x²) = 0
x= 0 или 9 - x² = 0
x = 0 или x² = 9 ⇒ x= -3 или x= 3
ответ: -3, 0, 3
3) x³ + 3x² - 4x - 12 = 0 
(x³ + 3x²) - (4x + 12) = 0
x²(x + 3) - 4(x + 3) = 0
(x² - 4)(x + 3) = 0
x² - 4 = 0 или x+3 = 0
x=2 или x=-2 или x=-3
4) x³ - 2x² = 0 
x^2(x - 2) = 0
x = 0 или x=2
5) 9x² - 0.25 = 0
9x² = 0.25
x² = 1/36
x = 1/6 или x=-1/6
6) 64 - 9x² = 0
9x² = 64
x² = 64/9
x = 8/3 или x= - 8/3
7) 2x³ - x² - 18x + 9 = 0
(2x³ - x²) - (18x - 9) = 0
x²(2x - 1) - 9(2x - 1) = 0
(x² - 9)(2x - 1) = 0
x²=9 или 2x - 1 = 0
x = -3 или x= 3 или x = 1/2
8) x² - 6x + 9 = 0
(x - 3)² = 0
x - 3 = 0
x = 3
9) x²+10x+25=0
(x + 5)² = 0
x + 5 = 0
x = -5
10) 4x² - 4x + 1 = 0
(2x - 1)² = 0
2x - 1 = 0
2x = 1
x = 1/2
11) 9x²+6x+1=0
(3x + 1)² = 0
3x +1 = 0
x = -1/3
12) 3x - 0.4x²=0
x(3 - 0.4x) = 0
x = 0 или 3-0.4x = 0 ⇒ (2/5)x = 3 ⇒ x = 15/2 = 7.5
13) -0.4x² + 8x = 0
x( 8 - 0.4x) = 0
x = 0 или 0,4x = 8 ⇒ x = 20