函数的主义域为{x+1>0
{2x+4>0
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{x>-1
{x>-2 所以x>-1
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g-f=log2[(2x+4)^2/(x+1)] =log[4*(x+2)^2]/(x+1)=log2(4)+log2[(x+2)^2/(x+1)]
=2+log2{[(x+1)+1]^2/(x+1)}
{[(x+1)+1]^2/(x+1)}=(x+1)+1/(x+1)+2≥2√(x+1)*[1/(x+1)]+2=4
所以g-f≥2+log2{4}=2+2=4