Tự chứng minh từng cái này rồi suy ra cái đó nhé b.

Ta có: \(sin\frac{A}{2}cos\frac{B}{2}cos\frac{C}{2}-sin\frac{A}{2}sin\frac{B}{2}sin\frac{C}{2}=sin^2\frac{A}{2}\)

Tương tự ta suy ra: 

\(sin\frac{A}{2}cos\frac{B}{2}cos\frac{C}{2}+cos\frac{A}{2}sin\frac{B}{2}cos\frac{C}{2}+cos\frac{A}{2}cos\frac{B}{2}sin\frac{C}{2}=sin^2\frac{A}{2}+sin^2\frac{B}{2}+sin^2\frac{C}{2}+3sin\frac{A}{2}sin\frac{B}{2}sin\frac{C}{2}\left(1\right)\)

Tiếp theo chứng minh:

\(2sin\frac{A}{2}sin\frac{B}{2}sin\frac{C}{2}=\frac{cosA+cosB+cosC-1}{2}\left(2\right)\)

\(sin^2\frac{A}{2}+sin^2\frac{B}{2}+sin^2\frac{C}{2}=\frac{3}{2}-\frac{cosA+cosB+cosC}{2}\left(3\right)\)

\(tan\frac{A}{2}tan\frac{B}{2}+tan\frac{B}{2}tan\frac{C}{2}+tan\frac{C}{2}tan\frac{A}{2}=1\left(4\right)\)

Từ (1), (2), (3), (4) suy được điều phải chứng minh

ko hiểu ( vì em mới học lớp 6)

Bài 14.

Áp dụng định lí hàm số Cô sin, ta có:

\(\dfrac{{{\mathop{\rm tanA}\nolimits} }}{{\tan B}} = \dfrac{{\sin A.\cos B}}{{\cos A.\sin B}} = \dfrac{{\dfrac{a}{{2R}}.\dfrac{{{c^2} + {a^2} - {b^2}}}{{2ac}}}}{{\dfrac{b}{{2R}}.\dfrac{{{c^2} + {b^2} - {a^2}}}{{2bc}}}} = \dfrac{{{c^2} + {a^2} - {b^2}}}{{{c^2} + {b^2} - {a^2}}} \)

Bài 19.

Áp dụng định lí sin và định lí Cô sin, ta có:

\( \cot A + \cot B + \cot C\\ = \dfrac{{R\left( {{b^2} + {c^2} - {a^2}} \right)}}{{abc}} + \dfrac{{R\left( {{c^2} + {a^2} - {b^2}} \right)}}{{abc}} + \dfrac{{R\left( {{a^2} + {b^2} - {c^2}} \right)}}{{abc}} = \dfrac{{R\left( {{a^2} + {b^2} + {c^2}} \right)}}{{abc}}\left( {dpcm} \right) \)

a/ \(\frac{A}{2}+\left(\frac{B}{2}+\frac{C}{2}\right)=90^0\)

\(\Rightarrow sin\frac{A}{2}=cos\left(\frac{B}{2}+\frac{C}{2}\right)=cos\frac{B}{2}cos\frac{C}{2}-sin\frac{B}{2}.sin\frac{C}{2}\)

b/ \(\frac{tan^2A-tan^2B}{1-tan^2A.tan^2B}=\frac{\left(tanA-tanB\right)}{\left(1+tanA.tanB\right)}.\frac{\left(tanA+tanB\right)}{\left(1-tanA.tanB\right)}=tan\left(A-B\right).tan\left(A+B\right)\)

\(=tan\left(A-B\right).tan\left(180^0-C\right)=-tan\left(A-B\right).tanC\)

c/

\(A+B+C=180^0\Rightarrow cot\left(A+B\right)=-cotC\)

\(\Leftrightarrow\frac{cotA.cotB-1}{cotA+cotB}=-cotC\)

\(\Leftrightarrow cotA.cotB-1=-cotA.cotC-cotB.cotC\)

\(\Leftrightarrow cotA.cotB+cotB.cotC+cotA.cotC=1\)

Tại sao lại suy đc Tại sao lại suy đc c

a/ Có \(\sin B=\frac{AC}{BC};\sin C=\frac{AB}{BC};\cos B=\frac{AB}{BC};\cos C=\frac{AC}{BC}\)

\(\Rightarrow\frac{\sin B-\sin C}{\cos B-\cos C}=\frac{AC-AB}{AB-AC}\)

Nếu AC<AB=> AC-AB<0 =>...<0

Nếu AC>AB=>AB-AC<0=>...<0

b/ làm tg tự câu a

c/ \(\cot B=\frac{AB}{AC};\cot C=\frac{AC}{AB}\)

\(\Rightarrow\cot B+\cot C=\frac{AB^2+AC^2}{AB.AC}\)

Quy đồng lên có: \(AB^2+AC^2>2AB.AC\) (luôn đúng vs AB\(\ne\) AC)

Vậy đẳng thức đc CM

Xét tam giác ABC, ta có:

\(\widehat A + \widehat B + \widehat C = {180^o} \Rightarrow \frac{{\widehat A}}{2} + \frac{{\widehat B + \widehat C}}{2} = {90^o}\)

Do đó \(\frac{{\widehat A}}{2}\) và \(\frac{{\widehat B + \widehat C}}{2}\) là hai góc phụ nhau.

a) Ta có: \(\sin \frac{A}{2} = \cos \left( {{{90}^o} - \frac{A}{2}} \right) = \cos \frac{{B + C}}{2}\)

b) Ta có: \(\tan \frac{{B + C}}{2} = \cot \left( {{{90}^o} - \frac{{B + C}}{2}} \right) = \cot \frac{A}{2}\)

f/

\(sin2A+sin2B+sin2C=2sin\left(A+B\right).cos\left(A-B\right)+2sinC.cosC\)

\(=2sinC.cos\left(A-B\right)+2sinC.cosC\)

\(=2sinC\left(cos\left(A-B\right)+cosC\right)\)

\(=2sinC\left[cos\left(A-B\right)-cos\left(A+B\right)\right]\)

\(=4sinC.sinA.sinB\)

g/

\(cos^2A+cos^2B+cos^2C=\frac{1}{2}+\frac{1}{2}cos2A+\frac{1}{2}+\frac{1}{2}cos2B+cos^2C\)

\(=1+\frac{1}{2}\left(cos2A+cos2B\right)+cos^2C\)

\(=1+cos\left(A+B\right).cos\left(A-B\right)+cos^2C\)

\(=1-cosC.cos\left(A-B\right)+cos^2C\)

\(=1-cosC\left(cos\left(A-B\right)-cosC\right)\)

\(=1-cosC\left[cos\left(A-B\right)+cos\left(A+B\right)\right]\)

\(=1-2cosC.cosA.cosB\)

d/ \(sinA+sinB+sinC=2sin\frac{A+B}{2}cos\frac{A-B}{2}+2sin\frac{C}{2}.cos\frac{C}{2}\)

\(=2cos\frac{C}{2}.cos\frac{A-B}{2}+2sin\frac{C}{2}.cos\frac{C}{2}\)

\(=2cos\frac{C}{2}\left(cos\frac{A-B}{2}+sin\frac{C}{2}\right)\)

\(=2cos\frac{C}{2}\left(cos\frac{A-B}{2}+cos\frac{A+B}{2}\right)\)

\(=4cos\frac{C}{2}.cos\frac{A}{2}.cos\frac{B}{2}\)

e/

\(cosA+cosB+cosC=2cos\frac{A+B}{2}cos\frac{A-B}{2}+1-2sin^2\frac{C}{2}\)

\(=1+2sin\frac{C}{2}.cos\frac{A-B}{2}-2sin^2\frac{C}{2}\)

\(=1+2sin\frac{C}{2}\left(cos\frac{A-B}{2}-sin\frac{C}{2}\right)\)

\(=1+2sin\frac{C}{2}\left(cos\frac{A-B}{2}-cos\frac{A+B}{2}\right)\)

\(=1+4sin\frac{C}{2}.sin\frac{A}{2}sin\frac{B}{2}\)