-47
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x2 + 49 = 0 ∴ x2 = -49 ∴ x = 7i
x2 + 49 = x2 - (-49) = x2 - (-1)(49) = x2 - (i2)(72) = x2 - (7i)2 = (x - 7i)(x + 7i) where i is the imaginary square root of -1.
x2 - 14x = -49 ∴ x2 - 14x + 49 = 0 ∴ (x - 7)2 = 0 ∴ x = 7
x4-49 is the difference of two squares when factored as (x2-7)(x2+7)
x2+14x+49 = (x+7)(x+7) when factored