About Lesson
(1) Prove that 证明
(a)
(b)
(c)
(2) Without using any calculator or tables, find the value of
不用计算机或查表,求下列各式的值
(a) sin100° sin200° sin300° sin400° 的值。
(b) cos20º cos40º cos60º cos80º
Answer 答案:
(a)
![Rendered by QuickLaTeX.com \frac{3}{16}](https://learn-ondemand.com/wp-content/ql-cache/quicklatex.com-4b58b48e216b1f22b1d0f51b4c5dfd9e_l3.png)
(b)