thu nam tuan sau la ngay 31

\(\dfrac{sin^3\alpha+3cos^3\alpha}{27sin^3\alpha-25cos^3\alpha}\)

\(=\dfrac{\dfrac{sin^3\alpha}{c\text{os}^3\alpha}+\dfrac{3cos^3\alpha}{c\text{os}^3\alpha}}{\dfrac{27sin^3\alpha}{c\text{os}^3\alpha}-\dfrac{25cos^3\alpha}{c\text{os}^3\alpha}}\)

\(=\dfrac{tan\alpha+3}{27tan\alpha-25}\)

\(=\dfrac{\dfrac{2}{3}+3}{27.\dfrac{2}{3}-25}\)

\(=-\dfrac{11}{21}\)

Bán quyển sách đó với giá là:12000:80*100=15000[đồng]

                                                         Đáp số:15000 đồng