equ4 

y=x^4-10x^2+9==0;  (* biquadratiche *)
Solve[y,x]
{{x -> 3}, {x -> -3}, {x -> 1}, {x -> -1}}
;[o]
{{x -> 3}, {x -> -3}, {x -> 1}, {x -> -1}}

y=x^4-3x^2-4==0;
Solve[y,x]
{{x -> I}, {x -> -I}, {x -> 2}, {x -> -2}}
;[o]
{{x -> I}, {x -> -I}, {x -> 2}, {x -> -2}}
y=2x^4+5x^2+2==0;
Solve[y,x]
{{x -> I*2^(1/2)}, {x -> -I*2^(1/2)}, {x -> I/2^(1/2)}, {x -> -I/2^(1/2)}}

{{x -> I Sqrt[2]}, {x -> -I Sqrt[2]}, {x -> -------}, {x -> -------}}
                                            Sqrt[2]         Sqrt[2]
y=x^4-18x^2+81==0;
Solve[y,x]
{{x -> 3}, {x -> -3}, {x -> 3}, {x -> -3}}
;[o]
{{x -> 3}, {x -> -3}, {x -> 3}, {x -> -3}}
y=a^2 x^4-(a^4+1) x^2+a^2==0;
Solve[y,x]
{{x -> a}, {x -> -a}, {x -> a^(-1)}, {x -> -a^(-1)}}
;[o]
                            1           1
{{x -> a}, {x -> -a}, {x -> -}, {x -> -(-)}}
                            a           a
y=x^4-(a^4-a^2) x^2-a^6==0;
Solve[y,x]
{{x -> a^2}, {x -> -a^2}, {x -> I*a}, {x -> -I*a}}
;[o]
        2           2
{{x -> a }, {x -> -a }, {x -> I a}, {x -> -I a}}

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