Question

Find all the zeros (please show work).

x^4 - 8x^2 - 9 = 0

Answer 1

put x^2=y
y^2-8y-9=0
1*-9=-9
9-1=8
9*-1=-9
write -8y=-9y+1y
make factors and put y=x^2
again if possible make factors.

Answer 2

Put x^2 = X -> X must be positive
Solve X^2 - 8X - 9 = 0. Since a - b + c = 0, use the TIP 2 to get the 2 real roots:
X1 = -1 (rejected), and X = (-c/a) = 9
x^2 = X = 9-> x = -3 (rejected), and x = 3.