✔ 最佳答案
This is a very good question.
Make use of the fact that sin(2x) = 2 sin(x) cos(x),
then sin(x) cos(x) = (1/2) sin(2x).
Let S = cos20° cos 40° cos80°.
S is what you want.
Consider sin20° S = sin20°cos20° cos 40° cos80°
= (1/2) sin40° cos 40° cos80°
= (1/2)(1/2) sin80° cos80°
= (1/2)(1/2)(1/2) sin160°
= (1/2)(1/2)(1/2) sin(180° - 20°)
= (1/2)(1/2)(1/2) sin20°
S = (1/2)(1/2)(1/2) = 1/8
2013-08-23 00:44:03 補充:
其實這題已經出現在各大教科書之中了~
都幾得意既~
也要用到 sin(x) = sin(180° -x)
2013-08-23 22:26:31 補充:
cf, 現時中學的課程,只有M2才會教double angle formula(即以前Additional Mathematics教的)。
你咁講即係你應該有讀M2,加油! =^o^=