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X + 1 = Numero Siguiente
X - 1 = Numero Anterior
(X + 1)² - 20 = (X - 1)(X + 1)
X² + 2X + 1 - 20 = X² + X - X - 1
X² + 2X - 19 = X² - 1
X² + 2X - X² = - 1 + 19
2X = 18
X = 18/2
X = 9
Prueba
(9 + 1)² - 20 = 10² - 20 = 100 - 20 = 80
(9 - 1)(9 + 1) = (8)(10) = 80
80 = 80
Rta: El numero es 9