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x² + 6 x + 9 = 0

( x + 3 )²  = 0  ⇔  x = - 3

1 + 8 x + 16 x² = 0

( 4 x + 1 ) ² = 0    ⇔ x = - 1/4

x² - 2 x + 1 = 0

( x - 1 )² = 0  ⇔ x = 1

x² - 16 = 0

( x - 4 ) ( x + 4 ) = 0  ⇔ x = 4 ou - 4

25 x² - 9  = 0

( 5 x - 3 ) ( 5 x + 3 )=  0 ⇔ x =  3/5 ou - 3/5

9 x² - 100 = 0

( 3 x - 10 ) ( 3 x + 10 ) = 0  ⇔ x = 10/3 ou - 10/3