Question

7x²-21=0 b)-x²+9=0 c) x²-16=0

Answer 1

$7 x^{2} -21=0\to\\\\ 7 x^{2} =21\to \\\\ x^{2} = \frac{21}{7} \to \\\\ x^{2} =3\to\\\\ x=\pm \sqrt{3} \\\\\\ S=\{- \sqrt{3} ; \sqrt{3} \}$




$-x^2+9=0\to \\\\- x^{2} =-9(-1)\to\\\\ x^{2} =9\to\\\\ x=\pm \sqrt{9} \to\\\\ x=\pm 3\\\\\\ S=\{-3;3\}$



$x^{2} -16=0\to\\\\ x^{2} =16\to\\\\ x=\pm \sqrt{16} \to \\\\x=\pm4\\\\\\ S=\{-4;4\}$

Answer 2

a)
7x² - 21 = 0
7x² = 21
x² = 21/7
x² = 3
x = + √3 e x = - √3
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b)
- x² + 9 = 0
- x² = - 9
x² = 9
x = √9
R.: x = + 3 e x = - 3
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c)

x² - 16 = 0
x² = 16
x = √16
x = +/- 4
R.: x = + 4 e x = - 4