# Quod Erat Demonstrandum

## 2010/04/26

### 一對直線

Filed under: Additional / Applied Mathematics,Pure Mathematics — johnmayhk @ 4:09 下午
Tags: $(a_1x+ b_1y + c_1)(a_2x+ b_2y + c_2) = 0$ $(x + y)\log(x + y) = x + y - 1$

[OT] 午飯，學校門口有人派傳單，宣傳一個數學網站。其中一個口號是：「勢必揭起網上學習旋風」。上去看看，數學東西是有的，但感覺也是宣傳多於一切。假假地在下也有十多年所謂「網上教學」經驗，從來看不到「網上學習」（esp. math. stuff）在本地會產生什麼「旋風」，荒涼地「營運」或（半）休眠才是常態。

## 4 則迴響 »

1. Ha, long time not leaving comment here~

By simple calculus it can be shown that the equation t ln t – t + 1 = 0 has only one real solution, namely t = 1, even after allowing ln t to have complex value when t<0. So there should be one straight line x+y=1 plotted. I guess it is numerical error and the two equations plotted in Winplot is very close?

迴響 由 馬高迪克 — 2010/04/27 @ 1:58 上午 | 回應

• Thank you 馬高迪克! (ot: why change your ‘name’ into Chinese?)

Yes, xlnx – x + 1 = 0 has exactly one root (x = 1), but what I’d used in the Winplot was ‘log’ (instead of ‘ln’) and the fact that xlogx – x + 1 = 0 has a root 7.292411085… (other than 1)
resulting the graph of a pair of straight lines.

And it is still very strange to me…

P.S. Sorry, I can’t remember, are you staying postdoc now? Thank you again for leaving a message, have a nice day!

迴響 由 johnmayhk — 2010/04/27 @ 9:05 上午 | 回應

2. Actually I forget what name I usually use here. My MSN is MarcoDick, though. Sometimes I find the Chinese translation 馬高迪克 more 型, like 迪克與牛仔.

I am not that old, now being first year PhD student in NYU Computer Science; yet Mathematics is always in my heart.

迴響 由 馬高迪克 — 2010/04/27 @ 9:14 上午 | 回應

• Thank you 迪克博士 for having the heart of math and willing to share! ^_^

迴響 由 johnmayhk — 2010/04/27 @ 9:21 上午 | 回應