Respostas

2014-06-09T20:14:06-03:00
A) 7x²-21=0
7x²=21
x²=21/7
x²=3
x=√3

b) -x²+4x=0
-x(x+4)=0
x=0

x+4=0
x=4

c) 2x²+5x=0
x(2x+5)=0
x=0

2x+5=0
2x=5
x=5/2
1 5 1
  • Usuário do Brainly
2014-06-09T20:16:31-03:00
A) 7x² - 21 = 0

7x² = 21
x² = 21/7
x² = 3
x = ± √3

B) - x² + 4x = 0

- x² + 4x = 0
x . (- x + 4) = 0
x = 0
- x + 4 = 0
- x = - 4 . (- 1)
x = 4

C) 2x² + 5x = 0

2x² + 5x = 0
x . (2x + 5) = 0
x = 0
2x + 5 = 0
2x = - 5
x = - 5/2
1 5 1