Решение и ответ:

0.2 - 1.8x² = 0

1.8x² = 0.2

x²=0.2 ÷ 1.8

x² = 1/9

x = ±√1/9 = ±1/3

x₁ = 1/3

x₂ = -1/3

1/3x² - 3 = 0

1/3x² = 3

x² = 3÷ 1/3

x² = 9

x = ±√9 = ±3

x₁ = 3

x₂ = -3

7 - 4/7x² = 0

4/7х² = 7

x² = 7 ÷ 4/7

x² = 49/4

x = ±√​49/4 = ±7/2

x₁ = 7/2 = 3.5

x₂ = -7/2 = 3.5