若X

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若X若X若X证明:因为x5–4x>0=>(5–4x)+1/(5–4x)≥2√{(5–4x)*[1/(5–4x)]}=2=>1–4x+1/(5–4x)=(5–4x)+1/(5–4x)–4≥-2,(当且仅

若X
若X

若X
证明:因为x < 5/4,所以4x < 5 => 5 – 4x > 0 => (5 – 4x) + 1/(5 – 4x) ≥ 2√{(5 – 4x)*[1/(5 – 4x)]} = 2 => 1 – 4x + 1/(5 – 4x) = (5 – 4x) + 1/(5 – 4x) – 4 ≥ -2,(当且仅当(5 – 4x) = 1/(5 – 4x),即x = 1时取到-2),得证.