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Basic method – with the converse scalene triangle inequality

Basic method – with the converse scalene triangle inequality

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What’s the Count Theorem? Imagine if you may have a pair of triangles which have a few congruent sides however, a different sort of direction ranging from those individuals corners. Think of it as a beneficial depend, which have repaired edges, that can be exposed to various bases:

Brand new Rely Theorem states one to on the triangle the spot where the incorporated angle is actually big, along side it contrary so it angle is larger.

It is extremely both called the “Alligator Theorem” because you can consider the corners since the (repaired length) oral cavity from an enthusiastic alligator- this new wide they opens the lips, the larger the fresh victim it will match.


To show the new Rely Theorem, we should instead demonstrate that one line sector was larger than another. Each other outlines are corners into the good triangle. It books us to fool around with among the many triangle inequalities which render a love between sides from an effective triangle. One of them is the converse of scalene triangle Inequality.

It tells us the front up against the larger position try larger than the side against the smaller perspective. One other is the triangle inequality theorem, which tells us the sum of the people one or two corners regarding a triangle was larger than the next top.

However, one hurdle first: both these theorems handle edges (or angles) of one triangle. Here we have two separate triangles. Therefore, the first order from business is to find these sides towards the one triangle.

Let’s place triangle ?ABC over ?DEF so that one of the congruent edges overlaps, and since ?2>?1, the other congruent edge will be outside ?ABC:

The above description was a colloquial, layman’s description of what we are doing. In practice, we will use a compass and straight edge to construct a new triangle, ?GBC, by copying angle ?2 into a new angle ?GBC, and copying the length of DE onto the ray BG so that |DE=|GB|=|AB|.

We’ll now compare the newly constructed triangle ?GBC to ?DEF. We have |DE=|GB| by construction, ?2=?DEF=?GBC by construction, and |BC|=|EF| (given). So the two triangles are congruent by the Side-Angle-Side postulate, and as a result |GC|=|DF|.

Let’s glance at the very first means for indicating the fresh new Hinge Theorem. To place this new corners we must contrast from inside the a good solitary triangle, we will mark a column from Grams so you’re able to A good. So it models an alternative triangle, ?GAC. It triangle features top Ac, and you will regarding a lot more than congruent triangles, front |GC|=|DF|.

Now why don’t we take a look at ?GBA. |GB|=|AB| by the design, very ?GBA was isosceles. Throughout the Base Bases spdate hesap silme theorem, i’ve ?BGA= ?Purse. In the perspective inclusion postulate, ?BGA>?CGA, and also have ?CAG>?Wallet. So ?CAG>?BAG=?BGA>?CGA, and so ?CAG>?CGA.

And then, on converse of your scalene triangle Inequality, the side contrary the large position (GC) is larger than one opposite small position (AC). |GC|>|AC|, and since |GC|=|DF|, |DF|>|AC|

2nd strategy – using the triangle inequality

Into next style of exhibiting the fresh Depend Theorem, we’ll create a similar this new triangle, ?GBC, because the ahead of. But now, unlike hooking up G so you can An effective, we’ll mark the position bisector regarding ?GBA, and you will stretch they until it intersects CG during the section H:

Triangles ?BHG and ?BHA are congruent of the Front-Angle-Side postulate: AH is a type of top, |GB|=|AB| by construction and you may ?HBG??HBA, given that BH ‘s the direction bisector. This means that |GH|=|HA| because the involved corners inside congruent triangles.

Today envision triangle ?AHC. In the triangle inequality theorem, i’ve |CH|+|HA|>|AC|. However, due to the fact |GH|=|HA|, we can replacement and also have |CH|+|GH|>|AC|. However, |CH|+|GH| is actually |CG|, thus |CG|>|AC|, and as |GC|=|DF|, we get |DF|>|AC|

And therefore we had been capable show new Rely Theorem (called the latest Alligator theorem) in 2 implies, counting on new triangle inequality theorem or their converse.

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