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secondary 4 | E Maths
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How to do part (iii)? Does it relate to part (I) and (ii) at all?
We need to do some calculations.
Will type out later at ~ 2 am.
If HO would be parallel to FB, it would make things simpler. But nowhere in the question mentioned this.
I give you a hint: it's indeed parallel. But how do we show this? (It's a prelude to the Midpoint Theorem)
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Consider triangles CHO and CFB.
• Notice clearly that CF = 2 CH because we are given that H is the midpoint of CF.
• Similarly, CB = 2 CO because O is the midpoint of the chord CB (meaning to say, both CO and OB are radii of the circle)
• So, the length ratio CF/CH = CB/CO = 2/1.
• With the sandwiched angle at C being a common angle, we have completed the S-A-S similarity proof.
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With the two triangles being similar, we will obtain the following result.
• Angle CHO = Angle CFB
But this would fall under the good corresponding angle rule - which can only happen when HO is parallel to FB.
[So, the converse of the corresponding angle rule will be true as well]
Now we will need this for our proof.
Will update this in the next post.
The subsequent result HO // FB is just as important.
Looking at triangles EXO and EFB, we will see several instances of corresponding angles
• Angle EXO = Angle EFB [A]
• Angle EOX = Angle EBF [A]
[Needless to say, angle E is common]
This completes our A-A similarity proof for triangles EXO and EFB.
Now, it's time to do the calculations.
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(First point)
First, what can we tell about the length ratio FB : HO?
Well, we have already found that triangles CFB and CHO are similar, with length ratio 2 : 1.
By default, FB : HO would also be 2 : 1.
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(Second point)
Observe that O is the midpoint of CB and E is the midpoint of CO. This means that
• CB = 2 CO
• CO = 2 EO
• EO : OB = 1 : 2
• EO : EB = 1 : 3
So, the length ratio of the corresponding lengths in triangles EXO and EFB are always 1 : 3.
• XO : FB = 1 : 3
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(Final result)
Let's combine the overall results.
• FB : HO = 2 : 1 = 6 : 3
• XO : FB = 1 : 3 = 2 : 6
As we know it too well,
• XO : FB : HO = 2 : 6 : 3
• XO : HO = 2 : 3
• OX : OH = 2 : 3
Now, it's clear that
• OX : XH = 2 : (3 - 2) = 2 : 1
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You can further truncate any unnecessary step(s) in my working.