A=3x4+5x2y2+2y4+2y2 biết x2+y2=2
A=3x2.x2+3x2y2+2.x2y2+y2.y2+2y2
A=3x2(x2+y2)+2.y2(x2+y2)+2y2
A=3x2.2+2y2.2+2y2
A=6x2+2y2(2+1)
A=6x2+2y2.3
A=6x2+6y2 =6(x2+y2)=6.2=12
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A=3x4+5x2y2+2y4+2y2
biết x2+y2=2 => x2=2-y2
=3(2-y2)2+5y2(2-y2)+2y4+2y2
=3(4-4y2+y4)+10y2-5y4+2y4+2y2
=12-12y2+3y4+12y2-3y4
=12