M=(x^4+1)(y^4+1)= x^4 +y^4 +(x*y)^4 +1
ap dung BDT cosi cho 2 so duong x^4 va y^4, ta co : x^4 + y^4>=2(x*y)^2
dau bang xay ra khi x^4=y^4 =>x^2=y^2<=> x=y hoac x=-y(loai)=>x=y=(can bac hai cua 10)/2
khi do M= x^4 +y^4 +(x*y)^4 +1>=2(x*y)^2 +(x*y)^4 +1=841/16