ข้อ 7 ประเภทบุคคล
$(x+y)(x^2-xy+y^2)=1957 ---------(1)$
$(x+y)(xy+x+y+1)=2014$
$(x+y)(3xy+3x+3y+3)=6042-------(3)$
$(1)+(3);$
$(x+y)(x^2+2xy+y^2+3x+3y+3)=7999$
$(x+y)[(x+y)^2+3(x+y)+3]=7999$
ให้ $A=x+y$
$A(A^2+3A+3)=7999$
$A^3+3A^2+3A=7999$
$A^3+3A^2+3A+1=8000$
$(A+1)^3=8000$
$A+1=20$
$A=19$
$\therefore x+y=19$
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