`a) (x+y+z).(x-y+z).(x+y-z).(x-y-z)`
`=[(x+y+z).(x-y-z)].[(x-y+z).(x+y-z)]`
`={[x+(y+z)].[x-(y+z)]}.{[x-(y-z)].[x+(y-z)]}`
`=[x^2 -(y+z)^2 ] .[x^2 -(y-z)^2 ]`
`=x^4 -[x(y+z)]^2 -[x(y-z)]^2 +[(y+z)^2 .(y-z)^2 ]`
`=x^4 -(xy++zy)^2 -(xy-yz)^2 +[(y+z).(y-z)]^2`
`=x^4 -x^2 y^2 -2xy^2 z -z^2 y^2 -x^2 y^2 +2xy^2 z -y^2 z^2 +(y^2 -z^2 )^2`
`=x^4 -2x^2 y^2 -2z^2 y^2 +y^4 -2y^2 z^2 +z^4`
`=x^4 +y^4 + z^4 -2x^2 y^2 -4y^2 z^2`
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