Câu 1:
\(a.sin\left(B-C\right)=a.sinBcosC-a.cosB.sinC\)
\(bsin\left(C-A\right)=bsinC.cosA-bcosC.sinA\)
\(csin\left(A-B\right)=csinAcosB-csinB.cosA\)
Cộng lại:
\(VT=cosA\left(bsinC-c.sinB\right)+cosB\left(c.sinA-a.sinC\right)+cosC\left(a.sinB-bsinA\right)\)
\(=cosA\left(\frac{b.c}{2R}-\frac{bc}{2R}\right)+cosB\left(\frac{ac}{2R}-\frac{ac}{2R}\right)+cosC\left(\frac{ab}{2R}-\frac{ab}{2R}\right)=0\)
Câu 2:
\(sin^2A+sin^2B+sin^2C=\frac{1}{2}-\frac{1}{2}cos2A+\frac{1}{2}-\frac{1}{2}cos2B+1-cos^2C\)
\(=2-\frac{1}{2}\left(cos2A+cos2B\right)-cosC.cosC\)
\(=2-cos\left(A+B\right)cos\left(A-B\right)+cosC.cos\left(A+B\right)\)
\(=2+cosC.cos\left(A-B\right)+cosC.cos\left(A+B\right)\)
\(=2+cosC\left[cos\left(A-B\right)+cos\left(A+B\right)\right]\)
\(=2+2cosA.cosB.cosC\)
Câu 3:
Ta có \(sin^2\frac{A}{2}=\frac{1-cosA}{2}=\frac{1-\frac{b^2+c^2-a^2}{2bc}}{2}=\frac{a^2-b^2-c^2+2bc}{4bc}=\frac{a^2-\left(b-c\right)^2}{4bc}\)
\(=\frac{\left(a+b-c\right)\left(a+c-b\right)}{4bc}=\frac{\left(p-c\right)\left(p-b\right)}{bc}\Rightarrow sin\frac{A}{2}=\sqrt{\frac{\left(p-b\right)\left(p-c\right)}{bc}}\)
Tương tự ta có \(sin\frac{B}{2}=\sqrt{\frac{\left(p-a\right)\left(p-c\right)}{ac}}\) ; \(sin\frac{C}{2}=\sqrt{\frac{\left(p-a\right)\left(p-b\right)}{ab}}\)
\(\Rightarrow4Rsin\frac{A}{2}sin\frac{B}{2}sin\frac{C}{2}=4\left(\frac{abc}{4S}\right)\sqrt{\frac{\left(p-a\right)^2\left(p-b\right)^2\left(p-c\right)^2}{a^2b^2c^2}}\)
\(=\frac{abc.\left(p-a\right)\left(p-b\right)\left(p-c\right)}{S.abc}=\frac{\left(p-a\right)\left(p-b\right)\left(p-c\right)}{S}=\frac{\left(p-a\right)\left(p-b\right)\left(p-c\right)}{\sqrt{p\left(p-a\right)\left(p-b\right)\left(p-c\right)}}=\sqrt{\frac{\left(p-a\right)\left(p-b\right)\left(p-c\right)}{p}}=r\)
