平方差公式，81(x+y)^4-16(x-y)^4=[9(x+y)^2+4(x-y)^2]*[9(x+y)^2-4(x-y)^2]=……
81(x+y)^4-16(x-y)^4
=[9(x+y)^2+4(x-y)^2][9(x+y)^2-4(x-y)^2]]
=[9(x+y)^2+4(x-y)^2][3(x+y)+2(x-y)][3(x+y)-2(x-y)]
=(13x^2+10xy+13y^2)(5x+y)(x+5y)
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