Question

[Lê Anh Ngọc]

Cho tam giác ABC, chứng minh rằng:

a) \(Sin\dfrac{A}{2}+Sin\dfrac{B}{2}+Sin\dfrac{C}{2}\le\dfrac{3}{2}\)

b) \(SinA+SinB+SinC\le\dfrac{3\sqrt{3}}{2}\)

Answer 1

[Phạm Minh Quang]

Ta có: A = \(sin\dfrac{A}{2}+sin\dfrac{B}{2}+sin\dfrac{C}{2}=cos\dfrac{B+C}{2}+2sin\dfrac{B+C}{4}cos\dfrac{B-C}{4}\)

\(\Leftrightarrow A-2sin\dfrac{B+C}{4}cos\dfrac{B-C}{4}-cos^2\dfrac{B+C}{4}+sin^2\dfrac{B+C}{4}=0\)\(\Leftrightarrow A-2sin\dfrac{B+C}{4}cos\dfrac{B-C}{4}+2sin^2\dfrac{B+C}{4}-1=0\)

Δ' = \(cos^2\dfrac{B-C}{4}-2\left(A-1\right)\ge0\)

\(\Rightarrow A-1\le\dfrac{1}{2}\Leftrightarrow A\le\dfrac{3}{2}\)