Quod Erat Demonstrandum

2017/03/19

盛水水深

Filed under: Additional / Applied Mathematics,HKCEE,NSS — johnmayhk @ 12:43 下午
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常見初中數學題:

圓錐容器高 H 單位。容器內盛水,垂直倒置時水深 h 單位(Fig. 1),把其倒轉平放水平面後(Fig.2),求水深。

利用相似形體積比等於對應邊比之立方,不難得 k=\sqrt[3]{H^3-h^3},故水深為

(H-\sqrt[3]{H^3-h^3}) 單位。

早前同事出題:如果容器是橢圓體,同樣問題如何解決?

具體一點,參考下圖

容器形狀是橢圓 \frac{x^2}{a^2}+\frac{y^2}{b^2}=1 環繞 y-軸轉出來的旋轉體。

容器內盛水,水深 h 單位(Fig. 3)把容器沿 O 轉 90 度(Fig.4)(注:其實是沿 z-軸),求水深。 (more…)

2016/03/21

用D證trigo

Filed under: Additional / Applied Mathematics,NSS — johnmayhk @ 11:13 上午
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某 M2 題…

Prove the following identity

\cos^2x+\cos^2(x+y)-2\cos y\cos x\cos(x+y)=\sin^2y.

試試"D吓佢"… (more…)

2015/08/28

sum of 1/k^2 from 1 to infinity

Filed under: Additional / Applied Mathematics,Fun — johnmayhk @ 4:30 下午
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非正式地所謂證明 \displaystyle \sum_{k=1}^\infty \frac{1}{k^2}=\frac{\pi^2}{6}
…………………………………………….

首先要知,下式的根之總和(sum of roots)

ax^n+bx^{n-1}+\dots +cx+d=0 ………. (*)

-\frac{b}{a}(more…)

2015/08/16

某舊習題

Filed under: Additional / Applied Mathematics,Pure Mathematics — johnmayhk @ 4:25 下午
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不知大家曾否被數學題吸引?剛在書櫃找東西時,隨意翻閱我校某本過時的中學數學參考書(1942 年,第 5 版)

johnmayhk-tutorial-algebra-cover

偶見一道頗吸引我的中學數學題:

證明:

a+b+c=0

\frac{a^5+b^5+c^5}{5}=\frac{a^3+b^3+c^3}{3}\cdot \frac{a^2+b^2+c^2}{2}

我只是被那公整的模樣吸引,其實這是一道純數的基本習題。但舊書用的是應數的方法,見下: (more…)

2014/06/11

M2 某題

Filed under: Additional / Applied Mathematics,NSS — johnmayhk @ 9:06 上午
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M2 學生問以下一題

The slope at any point (x,y) of a curve is given by

\frac{dy}{dx}=y(2x+9).

If the curve lies above the x-axis, and it passes (0,8), find the equation of the curve.
(more…)

2014/01/01

HT

Filed under: Additional / Applied Mathematics,Fun,HKALE — johnmayhk @ 12:27 上午
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以下題目也見過數次,現在才知是 1981 年比利時數學競賽的某題目:

不停擲一枚公平硬幣,問以下哪一事件出現的機會較大?

(a) THT 比 TTT 先出現;
(b) TTT 比 THT 先出現。

(T = tail,H = head)

比如

HHTHHHHTHTHHHTT… 就是 (a) 其中一種情況;
HTTHHHTTHHTTTHT… 就是 (b) 其中一種情況;

不知諸君會否認為 (a) 和 (b) 出現之機會均等?

非也, (more…)

2012/07/25

根中根

Filed under: Additional / Applied Mathematics,HKCEE,NSS — johnmayhk @ 11:40 上午

易知

\sqrt{3+\sqrt{8}}=1+\sqrt{2}

\sqrt{3+\sqrt{7}}

卻「不能」如上例作「進一步運算」。

於是學生曾問,甚麼樣的無理數 (more…)

2012/03/16

好像是 cosine law

Filed under: Additional / Applied Mathematics,NSS — johnmayhk @ 5:05 上午

中學同學熟識 Cosine law:

a^2=b^2+c^2-2bc\cos A

等。

之前看過,也頗有「美感」的 law:

a^2+b^2-2ab\cos(C+\frac{\pi}{3})=b^2+c^2-2bc\cos(A+\frac{\pi}{3})=c^2+a^2-2ca\cos(B+\frac{\pi}{3})

同學,先試試證明吧。

暫時想不到 (more…)

2011/10/03

無題

Filed under: Additional / Applied Mathematics,HKCEE,NSS — johnmayhk @ 7:39 下午

整理一下 draft 內的東西。

上年中五 M2 考試,隨便擬一道基本題:

Refer to the figure, the shaded region shown is bounded by the curves y=\sin (2x) and y=2\cos x. Find the area of the shaded region.

我期望 (more…)

2011/10/01

小心切線

Filed under: Additional / Applied Mathematics,HKCEE,NSS — johnmayhk @ 9:02 上午
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一道題:

Find the equation of tangent to the curve

x^3+x^2y-2x^2+xy^2-2xy+x+y^3+y=0 ………. (*)

at (1,0).

機械式地,同學應可 (more…)

2011/08/19

某數算題

Filed under: Additional / Applied Mathematics,HKALE,NSS,Teaching — johnmayhk @ 6:34 下午

給學生 98 道「暑期」(注 1)概率題目,這是第 77 題:

If three tickets are chosen at random without replacement from a set of 6n tickets numbered respectively 1, 2,…, 6n, what is the probability that the sum of the numbers on the numbers on the chosen tickets is 6n?

現在講解一下。 (more…)

2011/08/17

還是數算

Filed under: Additional / Applied Mathematics,HKALE,NSS,Teaching — johnmayhk @ 11:29 下午

暑假補課(注 1)時,學生問:

There are 10 empty boxes. 5 balls are going to put one by one into a randomly selected box. Find the probability that two of the boxes each contains 2 balls.

習題給的解是: (more…)

2011/08/09

Past papers 1964, 1971

Filed under: Additional / Applied Mathematics,Fun,HKALE,Pure Mathematics — johnmayhk @ 11:38 上午


Matriculation Examination (1964)
Advanced Level

Pure Mathematics I,II
Applied Mathematics I,II


(more…)

2011/04/13

解微分方程

Filed under: Additional / Applied Mathematics,HKALE — johnmayhk @ 8:50 上午

[Hardsell 廣告腔] 解微分方程?用

http://www.wolframalpha.com/

啦!

y"+y=sin(x) (more…)

2011/03/24

use series instead of lhopital

Filed under: Additional / Applied Mathematics,HKALE,Pure Mathematics — johnmayhk @ 3:10 下午

利用洛必達法則計算

\displaystyle \lim_{x\rightarrow 0}(\frac{\sin^{-1}x}{x})^{\frac{1}{x^2}}

頗煩。

或以無窮級數,粗糙地 (more…)

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