已知a,b,c,d为正数,求证:1<(a/a+b+c)+(b/a+b+c)+(c/a+b+c+d)+(d/a+b+c+d)<2
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(a/a+b+c)+(b/a+b+c)+(c/a+b+c+d)+(d/a+b+c+d)>(a/a+b+c+d)+(b/a+b+c+d)+(c/a+b+c+d)+(d/a+b+c+d)=(a+b+c+d)/(a+b+c+d)=1,
(a/a+b+c)+(b/a+b+c)+(c/a+b+c+d)+(d/a+b+c+d)<(a/a+b)+(b/a+b)+(c/c+d)+(d/c+d)
=(a+b)/(a+b)+(c+d)/(c+d)=1+1=2
(a/a+b+c)+(b/a+b+c)+(c/a+b+c+d)+(d/a+b+c+d)<(a/a+b)+(b/a+b)+(c/c+d)+(d/c+d)
=(a+b)/(a+b)+(c+d)/(c+d)=1+1=2
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