大家都仲係唔明....
你會唔會走去見到4COS^3 X-3COS X就會用COS3X開始推-V-
如果係背既話我冇野講了....當佢OUT C吧
韓信 發表於 3-5-2009 09:27 http://www4.nakuz.com/bbs/images/common/back.gif
加油啊韓信,我知你a到
我諗我A唔到-_-
做PAST PAPER都覺得好淺
一做呢D騎呢野就摸不著頭腦
99年paper2
第8題(a)part
本帖最後由 Zend 於 3-5-2009 13:23 編輯
treated it as double angle formula
(who told you amaths does not require memorization? huh?)
and in Pmaths, you can form equation by using De Moivre's theorem ....
http://upload.wikimedia.org/math/b/2/9/b29923f7db2847c9f284814eca4e78a9.png
本帖最後由 cla981085 於 3-5-2009 14:38 編輯
大家都仲係唔明....
你會唔會走去見到4COS^3 X-3COS X就會用COS3X開始推-V-
如果係背既話我冇野講了....當佢OUT C吧
韓信 發表於 3-5-2009 09:27 http://www4.nakuz.com/bbs/images/common/back.gif
我會-v-..
有時L.H.S.唔得,,咪用R.H.S. lo..
有時我兩邊一齊唻..
如果要對付一d好複雜既數,,兩邊一齊計就好快ko..好好使的.
又或者如果你想靚仔d既,,想寫到由L.H.S.變到R.H.S.,你可以係草稿寫左個proof (from R.H.S. to L.H.S.)
之後般返去answer book時就調返轉..
例如:
以上係由我之前個proof調返轉的
- -呢個做學校試卷做過( PROVE COS3X定唔記得SIN3X)...跟住有個同學一野寫答案....
好似一分都冇...原來係佢阿哥叫佢背[] sosad
4COS^3 X-3COS X
COS2X=2COS^2X -1
4CO3^3X-3COSX = 2COSX(2COS^2X-1) -COSX
=2COSXCOS2X-COSX
=2 -cosx
=cos3x+cosx-cosx
=cos3x
from destruction to construction
By De Movire's theorem
(cosx+isinx)^3 = cosx^3 +3cos^2xsinx i +3cosxsin^2x (i)^2 +sin^3 x i ^3= cos3x+isin3x
We have
cos3x = cos^3x -3cosxsin^2x(i^2 = -1)
=cos^3x -3cosx(1-cos^2x) = cos^3x -3cosx+3cos^3x = 4cos^3x - 3cosx
sin3x = 3cos^2xsinx -sin^3x = 3(1-sin^2x)sinx-sin^3x = 3sinx-3sin^3x-sin^3x = 3sinx-4sin^3x
i^3 = -i
以前的DM THEOREM 係IN C 的,所以唔使叫你地PROVE NE 樣野
但現在係OUT-C 所以得閒出一題都唔出奇
我會-v-..
有時L.H.S.唔得,,咪用R.H.S. lo..
有時我兩邊一齊唻..
如果要對付一d好複雜既數,,兩邊一齊計就好快ko..好好使的.
又或者如果你想靚仔d既,,想寫到由L.H.S.變到R.H.S.,你可以係草稿寫左個proof (from R.H ...
cla981085 發表於 3-5-2009 14:27 http://www1.nakuz.com/bbs/images/common/back.gif
我想問你第一步點解要拆出黎先
如果你冇用cos3x去計4cos3x-3cosx你會唔會識咁拆?
你可能會寫左cos3x+3cos3x-cosx-2cosx
Zend 又用pure野屈人?
用埋那些可惡的:
(cosx + isinx)^n = R(x) + I(x)
又要這個 : e^(pi*i) = -1
上面開始有D已眉目
有D用PURE MATHS 有D直頭PROVE到
其實我諗MARKING果條友都係背出黎= ="所以諗黎哂氣
會考應該唔會出到咁