The equation 9x^4+5x^2-4=0 has to be solved. Let y = x^2, this gives the equation
9y^2 + 5y - 4 = 0
=> 9y^2 + 9y - 4y - 4 = 0
=> 9y(y + 1) - 4(y + 1) = 0
=> (9y - 4)(y + 1) = 0
=> y = 4/9 and y = -1
x^2 = 4/9
=> x = -2/3 and x = 2/3
x^2 = -1
=> x = -i and x = i
The solution of the equation 9x^4+5x^2-4=0 is x = -i, x = i, x = -2/3 and x = 2/3
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