lượng giác

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ngomaithuy93

[tex]a+c=2b[/tex]
\Leftrightarrow[tex]2R(sinA+sinC)=4RsinB[/tex]
\Leftrightarrow[tex]sinA+sinC=2sinB[/tex]
\Leftrightarrow[tex]2\sin\frac{A+C}{2}.\cos\frac{A-C}{2}=2.\sin\frac{B}{2}.cos\frac{B}{2}[/tex]
\Leftrightarrow[tex]\cos\frac{A-C}{2}=\cos\frac{B}{2}[/tex]
\Leftrightarrow[tex]A-C=B[/tex]
\Leftrightarrow[tex]A-(B+C)=0.[/tex]
Mà[tex] A+B+C=180^0[/tex]\Rightarrowđpcm.
Đúng ko vậy bạn, nếu sai thì thôi vậy!:D
 
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