Bình chọn giảmTừ giả thiết ta suy ra {4≤a<95≤b<86≤c≤7" role="presentation" style="font-size: 13.696px; display: inline; position: relative;">⎧⎩⎨4≤a<95≤b<86≤c≤7{4≤a<95≤b<86≤c≤7⇒(a−4)(a−9)+(b−5)(b−8)+(c−6)(c−7)≤0" role="presentation" style="font-size: 13.696px; display: inline; position: relative;">⇒(a−4)(a−9)+(b−5)(b−8)+(c−6)(c−7)≤0⇒(a−4)(a−9)+(b−5)(b−8)+(c−6)(c−7)≤0⇔a2+b2+c2−13(a+b+c)+118≤0" role="presentation" style="font-size: 13.696px; position: relative;">⇔a2+b2+c2−13(a+b+c)+118≤0⇔a2+b2+c2−13(a+b+c)+118≤0⇔a+b+c≥16" role="presentation" style="font-size: 13.696px; display: inline; position: relative;">⇔a+b+c≥16⇔a+b+c≥16Dấu bằng xảy ra ⇔a=4;b=5;c=7" role="presentation" style="font-size: 13.696px; display: inline; position: relative;">⇔a=4;b=5;c=7