$b=x^5+y^5=(x+y)^5-5xy(x+y)(x^2+xy+y^2)=1-5xy(x^2+2xy+y^2-xy)$$=1-5xy(1-xy)=1-5xy+5x^2y^2 \Rightarrow VP=9b+1=5(9x^2y^2-9xy+2)$
$a=x^3+y^3=(x+y)^3-3xy(x+y)=1-3xy\Rightarrow VT = 5a(a+1)=5(1-3xy)(2-3xy)=5(9x^2y^2-9xy+2)=VT$
(đpcm)
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