find all possible roots of the polynomial where x∈c:?

a)
Let f(x) = 2x³ + 5x² + 14x + 6
f(-1/2) = 0
Then, (2x + 1) is a factor of 2x³ + 5x² + 14x + 6.

2x³ + 5x² + 14x + 6 =0
(2x³ + x²) + (4x² + 2x) + (12x + 6) = 0
x²(2x + 1) + 2x(2x + 1) + 6(2x + 1) = 0
(2x + 1)(x² + 2x + 6) = 0
2x + 1 = 0  or  x² + 2x + 6 = 0
x = -1/2

(x² + 2x + 6 = 0 has no real roots because discriminant = -20 < 0)

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b)
8x⁴ = x
8x⁴ - x = 0
x(8x³ - 1) = 0
x[(2x)³ - 1³] = 0
x(2x - 1)(4x² + 2x + 1) = 0
x = 0  or  2x - 1 = 0  or  4x² + 2x + 1 = 0
x = 0  or  x = 1/2

(4x² + 2x + 1 = 0 has no real roots because discriminant = -12 < 0)

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c)
x²(4x² + 17) = 15
4x⁴ + 17x² - 15 = 0
(4x² - 3)(x² + 5) = 0
(2x + √3)(2x - √3)(x² + 5) = 0
2x + √3 = 0  or  2x - √3 = 0  or  x² + 5 = 0
x = -(√3)/2  or  x = (√3)/2

(x² + 5 = 0 has no real roots.)