Bài 9:
a: \(\dfrac{1-cosa+cos2a}{sin2a-sina}\)
\(=\dfrac{1-cosa+2cos^2a-1}{2\cdot sina\cdot cosa-sina}\)
\(=\dfrac{2\cdot cos^2a-cosa}{sina\left(2\cdot cosa-1\right)}=\dfrac{cosa\left(2\cdot cosa-1\right)}{sina\left(2\cdot cosa-1\right)}=\dfrac{cosa}{sina}=cota\)
b: \(\dfrac{sin2a}{1+cos2a}\cdot\dfrac{cosa}{1+cosa}\)
\(=\dfrac{2\cdot sina\cdot cosa}{1+2cos^2a-1}\cdot\dfrac{cosa}{1+cosa}\)
\(=\dfrac{2\cdot sina\cdot cosa}{2\cdot cos^2a}\cdot\dfrac{cosa}{1+cosa}\)
\(=\dfrac{sina}{cosa}\cdot\dfrac{cosa}{1+cosa}=\dfrac{sina}{1+cosa}\)
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