解：
1、=X^2-XY-X^2+9
=9-XY
2, =(a^2+b^2)(a^2-b^2)
=a^4-b^4
3,=[a-(2b-3c)][a+(2b-3c)]
=a^2-(2b-3c)^2
=a^2-4b^2-9c^2+12bc
4.=[x+(y-z)][x-(y-z)]
=x^2-(y-z)^2
=x^2-y^2-z^2+2yz
5.=(x^8+y^8)(x^4+y^4)(x^2+y^2)(x^2-y^2)
=(x^8+y^8)(x^4+y^4)(x^4-y^4)
=(x^8+y^8)(x^8-y^8)
=x^16-y^16
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