若x-y=l,x3-y3=4,则x13-y13=______.
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若x-y=l,x3-y3=4,则x13-y13=______.
∵x-y=l,x3-y3=4,
∴x3-y3=(x-y)(x2+xy+y2)=(x-y)[(x-y)2+3xy]=4,
∴1×(1+3xy)=4,
∴xy=1,
∴x2+y2=3,
∴x4+y4=(x3-y3)(x-y)+x3y+xy3=7,
∴x7-y7=(x3-y3)(x4+y4)+x4y3-x3y4=29,
∴x10+y10=(x3-y3)(x7-y7)+x3y7+x7y3=4×29+7=123,
又∵x13-y13=(x3-y3)(x10+y10)-x3y10+y3x10,
=4(x10+y10)+x3y3(x7-y7),
=4×123+29,
=521.
故答案为:521.
∴x3-y3=(x-y)(x2+xy+y2)=(x-y)[(x-y)2+3xy]=4,
∴1×(1+3xy)=4,
∴xy=1,
∴x2+y2=3,
∴x4+y4=(x3-y3)(x-y)+x3y+xy3=7,
∴x7-y7=(x3-y3)(x4+y4)+x4y3-x3y4=29,
∴x10+y10=(x3-y3)(x7-y7)+x3y7+x7y3=4×29+7=123,
又∵x13-y13=(x3-y3)(x10+y10)-x3y10+y3x10,
=4(x10+y10)+x3y3(x7-y7),
=4×123+29,
=521.
故答案为:521.
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