Solve x4 – 17x2 + 16 = 0. let u = this is a question on my test

Solve for x over the real numbers:
x^4 - 17 x^2 + 16 = 0
Substitute y = x^2:
y^2 - 17 y + 16 = 0
The left hand side factors into a product with two terms:
(y - 16) (y - 1) = 0
Split into two equations:
y - 16 = 0 or y - 1 = 0
Add 16 to both sides:
y = 16 or y - 1 = 0
Substitute back for y = x^2:
x^2 = 16 or y - 1 = 0
Take the square root of both sides:
x = 4 or x = -4 or y - 1 = 0
Add 1 to both sides:
x = 4 or x = -4 or y = 1
Substitute back for y = x^2:
x = 4 or x = -4 or x^2 = 1
Take the square root of both sides:
x = 4 or x = -4 or x = 1 or x = -1

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