moi nguoi giep em voi. Bt kho qua

T

tranvanhung7997


\[Ta có: VT= sin^2A + sin^2B + sin^2C\]
\[=\frac{1-cos2A}{2} + \frac{1-cos2B}{2} + sin^2C \]
\[=1-\frac{1}{2}.(cos2A+cos2B)+sin^2C\]
\[=1-cos(A+B).cos(A-B)+ sin^2C\]
\[=1+cosC.cos(A-B)+1-cos^2C \]
\[=2+cosC.[cos(A-B)-cosC] \]
\[=2+cosC.[cos(A-B) + cos(A+B)] \]
\[=2+2.cosA.cosB.cosC = VP => đpcm. \]
 
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