x²y²-y²-x²+1
=(x²y²-y²)-(x²-1)
=y²(x²-1)-(x²-1)
=(x²-1)(y²-1)
=(x+1)(x-1)(y+1)(y-1)
=(x²y²-x²)-(y²-1)
=x²(y²-1)-(y²-1)
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