Nhận xét: \(\forall \Delta ABC\) ta có \(cos^2A+cos^2B+cos^2C+2cosA.cosB.cosC=1\) Từ (1) ta có \(a, b, c\in (0;2)\) Đặt \(a=2cosA, b=2cosB, c=2cosC\) Khi (1) trở thành \(cos^2A+cos^2B+cos^2C+2cosA.cosB.cosC=1\Rightarrow A,B,C\) là góc \(\Delta\) \(a+b+c=2cosA+2cosB+2cosC\) \(=4cos\frac{A+B}{2}.cos\frac{A-B}{2}+2(1-2sin^2\frac{C}{2})\) \(=4sin\frac{C}{2}.cos\frac{A-B}{2}-4sin^2\frac{C}{2}+2\) \(a+b+c\leq 3\) \(\Leftrightarrow -4sin^2\frac{C}{2}+4sin\frac{C}{2}.cos\frac{A-B}{2}-1\leq 0\) \(\Leftrightarrow -[4sin^2\frac{C}{2}-4sin\frac{C}{2}.cos\frac{A-B}{2}+ cos^2\frac{A-B}{2}]-sin^2\frac{A-B}{2}\leq 0\) \(\Leftrightarrow -(2sin\frac{C}{2}-cos\frac{A-B}{2})^2-sin^2\frac{A-B}{2}\leq 0\) đúng Dấu đẳng thức xảy ra khi \(A=B=C=60^0\Leftrightarrow a=b=c=1\)