M=(x²+1)²(x²-1)²=[(x²+1)(x²-1)]²=(x^4-1)²=x^8-2x^4+1N=(x^4+x^2+1)(x^4-x^2+1)=[(x^4+1)+x^2][(x^4+1)-x^2]=(x^4+1)²-x^4=x^8+x^4+1M-N=x^8-2x^4+1-(x^8+x^4+1)=-3x^4 <0(∵x≠0)∴M<N