引用:
原帖由 rchan626 於 2020-11-3 09:59 PM 發表
我既計法都係用相同扣減方法, 計到大約淨下 [12-5k+y(2k-2.4)]/2 就停左, 無辦法變到問題所問既 (5k/12)(12-5k).
I'm not sure how exactly did you get to that expression, maybe there are some algebra errors or something.
Now let me rephrase ehbb's method a little bit, so it "looks" nicer.
Suppose AE = x, EF = y, FC = 5-x-y
How about instead of thinking the area of the rectangle, think about the big triangle ABC?
[It doesn't make a difference, but adding things up instead of subtracting things away "feels" nicer]
One way of expressing the area of ABC is, well, the obviously way - it's just 3*4/2 = 6.
The other way is adding up all the 4 parts - 3 triangles and a rectangle. ehbb did a great job as to what the 4 small parts' areas are.
The trickiest one is DBG, where the base is DG, and the height is what you get from (a) minus k [Why? Pause and ponder]
Therefore, the long chain of adding up areas should equal to 6, which then you have to simplify...
Now don't forget our goal, we want to know what EF (i.e. y) is in terms of k.
I still prefer utilizing similar triangles more, but this approach works.
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本帖最後由 風之男 於 2020-11-3 10:58 PM 編輯 ]