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su dung bdt am-gm ta co :x^4+1+1+1lon hon hoac bang bon nhan can x^4=4nhan gia tri tuyet doi cua x luon lon hon hoac bang 4nhanx,Tuong tu y^4 +1+1+1 lon hon hoac bang bon nhan can y^4=4nhan gia tri tuyet doi cua y luon lon hon hoacbang 4nhan -y tu day suy ra x^4+y^4+6 lon hon hoac bang 4(x-y) suy ra (x-y) tren (x^4+y^4+6) nho hon hoac bang 4 Dau = xay ra khi x=1; y=-1
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