Question

7x² - 21=0

- x² + 4x=0

2x² + 5x=0

Answer 1

a) 7x² - 21=0
    7x² = 21
      x² = 21/7
      x² = 3
       x = +- √3

b) - x² + 4x=0
     x(-x+4)=0
     x = 0 ou x = 4

c) 2x² + 5x=0
     x(2x+5)=0
        x = 0  ou x=-5/2

Answer 2

1) 7x² - 21=0 => 7x² = 21 => x² = 21/7 => x² = 3 => x = √3
S{+√3, -√3}
.........
2) - x² + 4x=0 => x(-x + 4) = 0
x = 0
-x = -4 (-1) => x = 4
S{0, 4}
..........
3) 2x² + 5x=0 => x(2x+5) = 0
x = 0
2x+5 = 0 => 2x = -5 => x = -5/2
S{-5/2, 0}