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\(\dfrac{b^2-a^2}{2c}=b.\dfrac{\left(b^2+c^2-a^2\right)}{2bc}-a.\dfrac{\left(a^2+c^2-b^2\right)}{2ac}\)
\(\Leftrightarrow\dfrac{b^2-a^2}{2c}=\dfrac{b^2+c^2-a^2}{2c}-\dfrac{a^2+c^2-b^2}{2c}\)
\(\Leftrightarrow b^2-a^2=\left(b^2+c^2-a^2\right)-\left(a^2+c^2-b^2\right)\)
\(\Leftrightarrow3b^2=3a^2\Leftrightarrow a=b\)
Hay tam giác cân tại C
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\overrightarrow{BA}=\left(3;0\right)\Rightarrow AB=3=AC\) ; \(\overrightarrow{AC}=\left(a-2;b+2\right)\) ; \(\overrightarrow{BC}=\left(a+1;b+2\right)\)
\(BC=\sqrt{AB^2+AC^2-2AB.AC.cosA}=\dfrac{6\sqrt{5}}{5}\)
\(\Rightarrow\left\{{}\begin{matrix}\left(a-2\right)^2+\left(b+2\right)^2=9\\\left(a+1\right)^2+\left(b+2\right)^2=\dfrac{36}{5}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\left(a;b\right)=\left(\dfrac{1}{5};-\dfrac{22}{5}\right)\\\left(a;b\right)=\left(\dfrac{1}{5};\dfrac{2}{5}\right)\end{matrix}\right.\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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' = \(cos^2\dfrac{B-C}{4}-2\left(A-1\right)\ge0\)
\(\Rightarrow A-1\le\dfrac{1}{2}\Leftrightarrow A\le\dfrac{3}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) ta có : \(cos^2\left(a-b\right)-sin^2\left(a+b\right)\)
\(=\left(cosa.cosb+sina.sinb\right)^2-\left(sina.cosb+sinb.cosa\right)^2\)
\(=cos^2a.cos^2b+sin^2a.sin^2b-sin^2a.cos^2b-sin^2b.cos^2a\)
\(=cos^2a.cos^2b-sin^2a.cos^2b+sin^2a.sin^2b-sin^2b.cos^2a\)\(=cos^2b\left(cos^2a-sin^2a\right)-sin^2b\left(cos^2a-sin^2a\right)\)
\(=\left(cos^2b-sin^2b\right)\left(cos^2a-sin^2a\right)=cos2a.cos2b\left(đpcm\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1.
\(sinA+sinB-sinC=2sin\dfrac{A+B}{2}.cos\dfrac{A-B}{2}-sin\left(A+B\right)\)
\(=2sin\dfrac{A+B}{2}.cos\dfrac{A-B}{2}-2sin\dfrac{A+B}{2}.cos\dfrac{A+B}{2}\)
\(=2sin\dfrac{A+B}{2}.\left(cos\dfrac{A-B}{2}-cos\dfrac{A+B}{2}\right)\)
\(=2sin\dfrac{A+B}{2}.2sin\dfrac{A}{2}.sin\dfrac{B}{2}\)
\(=4sin\dfrac{A}{2}.sin\dfrac{B}{2}.cos\dfrac{C}{2}\)
Sao t lại đc như này v, ai check hộ phát
![](https://rs.olm.vn/images/avt/0.png?1311)
1.
Sửa đề: \(S=\dfrac{1}{6}\left(ch_a+bh_c+ah_b\right)\)
\(a.h_a=b.h_b=c.h_c=2S\Rightarrow\left\{{}\begin{matrix}h_a=\dfrac{2S}{a}\\h_b=\dfrac{2S}{b}\\h_c=\dfrac{2S}{c}\end{matrix}\right.\)
\(\Rightarrow6S=\dfrac{2Sc}{a}+\dfrac{2Sb}{c}+\dfrac{2Sa}{b}\)
\(\Leftrightarrow\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}=3\)
Mặt khác theo AM-GM: \(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\ge3\sqrt[3]{\dfrac{abc}{abc}}=3\)
Dấu "=" xảy ra khi và chỉ khi \(a=b=c\)
\(\Leftrightarrow\) Tam giác đã cho đều
2.
Bạn coi lại đề, biểu thức câu này rất kì quặc (2 vế không đồng bậc)
Ở vế trái là \(2\left(a^2+b^2+c^2\right)\) hay \(2\left(a^3+b^3+c^3\right)\) nhỉ?
3.
Theo câu a, ta có:
\(VT=\dfrac{2S}{a}+\dfrac{2S}{b}+\dfrac{2S}{c}\ge\dfrac{18S}{a+b+c}=\dfrac{18.pr}{a+b+c}=9r\)
Dấu "=" xảy ra khi và chỉ khi \(a=b=c\)
Hay tam giác đã cho đều