Two lines are parallel if they are equidistant from one another. The following proves [tex]a || m[/tex]
[tex]Ratio = 1:3[/tex] means that:
[tex]BP=3PA[/tex]
[tex]BQ=3QC[/tex]
To prove that [tex]a || m[/tex] is to prove that side length SR and side length PQ are parallel because:
[tex]SR = a\\ PQ = m[/tex] --------- See attachment for quadrilateral
Considering [tex]\triangle ABC[/tex], we have: [tex]BP=3PA[/tex]
Divide both sides by PA
[tex]\frac{BP}{PA} = \frac{3PA}{PA}[/tex]
[tex]\frac{BP}{PA} = 3[/tex]
Similarly: [tex]BQ=3QC[/tex]
Divide both sides by QC
[tex]\frac{BQ}{QC} = \frac{3BQ}{QC}[/tex]
[tex]\frac{BQ}{QC} = 3[/tex]
So, we have:
[tex]\frac{BP}{PA} = \frac{BQ}{QC} = 3[/tex]
Express as ratios
[tex]BP:PA = BQ:QC = 3[/tex]
[tex]BP:PA \to[/tex] means point P divided side length BA into 3:1
[tex]BQ:QC \to[/tex] means point Q divided side length BC into 3:1
Thales Theorem states that: A line that divides any two sides of a triangle in the same ratio is parallel to the third side.
The third side of [tex]\triangle ABC[/tex] is [tex]AC[/tex].
This means that: [tex]PQ[/tex] is parallel to [tex]AC[/tex]
Since [tex]AC[/tex] is parallel to [tex]SR[/tex], then [tex]PQ[/tex] is parallel to [tex]SR[/tex]
i.e. [tex]SR || PQ[/tex]
Recall that:
[tex]SR = a\\ PQ = m[/tex]
Hence:
[tex]a || m[/tex] has been proved.
Read more about proof of parallel sides at:
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