Answer :

x^2 - y^2 = 8
x^2 + y^2 = 4
Therefore:   (x^2 - y^2  ) * (x^2 + y^2 ) = 8 *4
Expanded:  x^4 + x^2 y^2 - x^2 y^2 - y^4 = 32
So: x^4 - y^4 = 32

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