Đáp án:
Giải thích các bước giải:
Ta có :
`a/[a+b+c]>a/[a+b+c+d]`
`b/[b+c+d]>b/[a+b+c+d]`
`c/[c+d+a]>c/[a+b+c+d]`
`d/[d+a+b]>d/[a+b+c+d]`
`=>a/[a+b+c]+b/[b+c+d]+c/[c+d+a]+d/[d+a+b]>a/[a+b+c+d]+b/[a+b+c+d]+c/[a+b+c+d]+d/[a+b+c+d]`
`=>a/[a+b+c]+b/[b+c+d]+c/[c+d+a]+d/[d+a+b]>[a+b+c+d]/[a+b+c+d]`
`=>a/[a+b+c]+b/[b+c+d]+c/[c+d+a]+d/[d+a+b]>1` `(1)`
Lại có :
`a/[a+b+c]<[a+d]/[a+b+c+d]`
`b/[b+c+d]<[b+a]/[a+b+c+d]`
`c/[c+d+a]<[c+b]/[a+b+c+d]`
`d/[d+a+b]<[d+c]/[a+b+c+d]`
`=>a/[a+b+c]+b/[b+c+d]+c/[c+d+a]+d/[d+a+b]<[a+d]/[a+b+c+d]+[b+a]/[a+b+c+d]+[c+b]/[a+b+c+d]+[d+c]/[a+b+c+d]`
`=>a/[a+b+c]+b/[b+c+d]+c/[c+d+a]+d/[d+a+b]<[2(a+b+c+d)]/[a+b+c+d]`
`=>a/[a+b+c]+b/[b+c+d]+c/[c+d+a]+d/[d+a+b]<2` `(2)`
Từ `(1),(2)=>1<a<2`
