Theo đề: : x - y = x^3 + y^3 > 0
ta có: x² + y² < 1
<=> ( x² + y² )( x - y ) < x^3 + y^3
<=> x^3 + y²x - x²y - y^3 < x^3 + y^3
<=> 2y^3 + x²y - y²x > 0
<=> 2y^3 + xy( x - y ) > 0 ( Luon dung do x , y duong va x - y > 0 )
=> DPCM