x³＋y³
＝﹙x＋y﹚﹙x²－xy＋y²﹚
=﹙x＋y﹚[﹙x＋y﹚²－3xy]
=9×﹙81－30﹚
=459
x+y=9
(x+y)²=9²
x²+y²+2xy=81
x²+y²=81-20=61
x3+y3
=(x+y)(x²-xy+y²)
=9×(61-10)
=9×51
=459
解：x³＋y³＝﹙x＋y﹚﹙x²－xy＋y²﹚
＝9[﹙x＋y﹚²－3xy]
＝9×﹙81－30﹚
＝459
x³+y³=(x+y)³-3xy(x+y)=9³-3*10*9=459
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