Question

Chứng minh rằng: \(\dfrac{\sin\alpha+\cos\alpha-1}{1-\cos\alpha}\)=\(\dfrac{2\cdot\cos\alpha}{\sin\alpha-\cos\alpha+1}\)

Answer 1

\(\Leftrightarrow\left(sina\right)^2-\left(cosa-1\right)^2=2cosa\left(1-cosa\right)\)

\(\Leftrightarrow1-cos^2a-cos^2a+2cosa-1=2cosa-2cos^2a\)

\(\Leftrightarrow-2cos^2a+2cosa=-2cos^2a+2cosa\)(đúng)