Find the solutions of the following equations.
a. 4x^4 − x^2 − 18 = 0
b. x^3 − 8 = 0
c. 8x^3 − 27 = 0
d. x^4 − 1 = 0
e. 81x^4 − 64 = 0
f. 20x^4 + 121x^2 − 25 = 0
g. 64x^3 + 27 = 0
h. x^3 + 125 = 0