cos20 cos 40 cos80

2013-08-22 10:51 pm
Without using the calculators, find the values of
cos20 cos 40 cos80

Please explain to me

回答 (4)

2013-08-22 11:02 pm
✔ 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^=
2013-08-24 4:34 pm
這個方法M2書本應該有教過啦@@ 如果你看不出來,其實直接用product to sum一樣可以,不過較為麻煩。

cos20ºcos 40ºcos80º
= ½(cos60º + cos20º)cos80º [i.e. cos60º = ½]
= ¼cos80º + ½cos20ºcos80º
= ¼cos80º + ¼(cos100º + cos60º) [i.e. cos100º = -cos80º]
= ¼cos60º
= 1/8 //
2013-08-24 6:21 am
勁~
M2都有教:D
2013-08-23 8:42 am
Nice trick :)


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