không dùng máy tính tính giá trị của biểu thức sau:

L

levietdung1998

\[\begin{array}{l}
C = c{\rm{os}}\frac{{3\pi }}{7} + c{\rm{os}}\frac{{9\pi }}{7} + c{\rm{os}}\frac{{15\pi }}{7} \\
= c{\rm{os}}\frac{{3\pi }}{7} + c{\rm{os}}\frac{{5\pi }}{7} + c{\rm{os}}\frac{\pi }{7} = c{\rm{os}}\frac{\pi }{7} + c{\rm{os}}\frac{{3\pi }}{7} + c{\rm{os}}\frac{{5\pi }}{7} \\
\to C.\sin \frac{\pi }{7} = \frac{1}{2}\sin \frac{{2\pi }}{7} + \frac{1}{2}\sin \frac{{4\pi }}{7} - \frac{1}{2}\sin \frac{{2\pi }}{7} + \frac{1}{2}\sin \frac{{6\pi }}{7} - \frac{1}{2}\sin \frac{{4\pi }}{7} \\
\to C.\sin \frac{\pi }{7} = \frac{1}{2}\sin \frac{{6\pi }}{7} \\
\to C\sin \frac{\pi }{7} = \frac{1}{2}\sin \frac{\pi }{7} \to C = \frac{1}{2} \\
\end{array}\]

“Bài dự thi event box toán 10”
 
You must log in or register to reply here.
Top Bottom