z^4-8z^2+15=0

Solution for z^4-8z^2+15=0 equation:

Simplifying
z4 + -8z2 + 15 = 0

Reorder the terms:
15 + -8z2 + z4 = 0

Solving
15 + -8z2 + z4 = 0

Solving for variable 'z'.

Factor a trinomial.
(3 + -1z2)(5 + -1z2) = 0

Subproblem 1
Set the factor '(3 + -1z2)' equal to zero and attempt to solve:

Simplifying
3 + -1z2 = 0

Solving
3 + -1z2 = 0

Move all terms containing z to the left, all other terms to the right.

Add '-3' to each side of the equation.
3 + -3 + -1z2 = 0 + -3

Combine like terms: 3 + -3 = 0
0 + -1z2 = 0 + -3
-1z2 = 0 + -3

Combine like terms: 0 + -3 = -3
-1z2 = -3

Divide each side by '-1'.
z2 = 3

Simplifying
z2 = 3

Take the square root of each side:
z = {-1.732050808, 1.732050808}
Subproblem 2
Set the factor '(5 + -1z2)' equal to zero and attempt to solve:

Simplifying
5 + -1z2 = 0

Solving
5 + -1z2 = 0

Move all terms containing z to the left, all other terms to the right.

Add '-5' to each side of the equation.
5 + -5 + -1z2 = 0 + -5

Combine like terms: 5 + -5 = 0
0 + -1z2 = 0 + -5
-1z2 = 0 + -5

Combine like terms: 0 + -5 = -5
-1z2 = -5

Divide each side by '-1'.
z2 = 5

Simplifying
z2 = 5

Take the square root of each side:
z = {-2.236067978, 2.236067978}
Solution
z = {-1.732050808, 1.732050808, -2.236067978, 2.236067978}