What is Triangle Inequality?
We have studied the equality of sides and angles between two triangles or in a triangle. Sometimes we may come across unequal objects or figures.
In our instances of comparisons, we take into consideration every part of the object. Similarly, when we compare various parts of a triangle, we compare every part individually. So let us study what is the triangle inequality. For instance, take the line segment shown in the figure:
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From the above figure, we can see that line segment AB is smaller than the line segment PQ. The difference is clear, yet to know the difference and confirm our findings we measure both of them. After measuring both, we come to the conclusion that they are unequal. Likewise, see the figure of angles below:
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The angles AOB and POQ are unequal. We can clearly see that ∠ POQ is greater than ∠AOB. These unequal figures have a close relationship between unequal sides and unequal angles of a triangle.
The term triangle inequality means unequal in their measures.
Let us consider any triangle of length AB, BC, and AC of three sides of a triangle. Then the triangle inequality definition or triangle inequality theorem states that
The sum of any two sides of a triangle is greater than or equal to the third side of a triangle.
i.e.,
AB + BC ≥ AC
Now let us understand the relation between the unequal sides and unequal angles of a triangle with the help of the triangle inequality theorems.
Triangle Inequality Theorem
Theorem 1: If two sides of a triangle are unequal, the longer side has a greater angle opposite to it.
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Triangle inequality theorem-proof:
Given: in \[\triangle ABC\] in which AC > AB
To prove :∠ ABC > ∠ ACB
Construction: Take a point D on AC such that AB = AD, Join BD.
Proof:
In \[\triangle ABC\], we have
AB = AD...by construction
∠ ABD = ∠ ADB….(1)......angle opposite to equal side are equal
Now, consider BCD, we find that ∠ADB is the exterior angle of BCD and an exterior angle is always greater than the interior opposite angle.
therefore,
∠ADB > ∠DCB
∠ ADB = ∠ACB…(2)
From 1 and 2 we get
∠ABD = ∠ ACB…..(3)
But ∠ABD is a part of ∠ABC
So, ∠ABC = ∠ABD…..(4)
From 3 and 4 we get
∠ABC > ∠ ACB……..thus proved
Theorem 2: (Converse of Theorem 1) in a triangle the greater angle has the longer side opposite to it.
Triangle inequality theorem proof
Given: In \[\triangle ABC\] ∠ ABC > ∠ ACB
To prove: AC > AB
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Proof:
In \[\triangle ABC\], we have the following three possibilities
AC =AB
AC < AB
AC > AB
Case 1 : When AC = AB
AC = AB
∠ ABC = ∠ ACB...angles equal to opposite sides are equal
This is contradiction
Since given that ∠ ABC > ∠ ACB
AC \[\neq\] AB
Case 2: when AC < AB
AC < AB
∠ ACB > ∠ ABC……..[Longest side has the greater angle opposite to it]
This also contradicts the given hypothesis.
Thus we are left with the only one possibility, AC > AB,
Hence AC > AB
Theorem 3: The sum of any two sides of a triangle is greater than the third side of a triangle.
Given: in \[\triangle ABC\]
To prove: AB + AC > BC,
AB + BC > AC and
BC + AC > AB
Construction: produce side BA to D such that AD = AC. join CD
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Proof:
in \[\triangle ACD\], we have
AC = AD ...by construction
∠ ADC = ∠ ACD…..angle opposite to equal sides are equal
∠ ACD = ∠ ADC
∠ BCA + ∠ ACD > ∠ ADC ……[ ∠ BCA + ∠ ACD > ∠ ACD]
∠ BCD > ∠ ADC
∠ BCD > ∠ BDC………………..[because ∠ ADC = ∠ BDC]
BD > BC …………………………[ side opposite to greater angle is larger]
BA + AD > BC
BA + AC > BC……………………[ since AC = AD by construction]
AB + AC > BC
Thus, AB + AC > BC
Similarly, we can prove AB + BC > AC and BC + AC > AB
Triangle Inequality Formula
In \[\triangle ABC\]
AB + AC > BC,
AB + BC > AC and
BC + AC > AB
Solved Examples
Example 1 : if side lengths 8cm, 5cm and 7cm are given can it form a triangle?
Solution: If 8cm, 5cm and 7cm are the sides of the triangle, then they should satisfy the inequality theorem.AB + AC > BC,
AB + BC > AC and
BC + AC > AB
Let AB = 8cm, BC = 5cm and AC = 7cm
Then
AB + BC > AC
8 + 5 > 7
⇒ 13 > 7
BC + AC > AB
5 + 7 > 8
⇒ 12 > 8
AB + AC > BC,
8 + 7 > 5
⇒ 15 > 5
All the three conditions are satisfied, therefore a triangle can be formed with sides 8cm, 5cm and 7cm.
Quiz Time
if side lengths 18cm, 5cm and 4cm are given can it form a triangle?
if side lengths 7cm, 15cm and 9cm are given can it form a triangle?
FAQs on Triangle Inequality
1. What are Congruent Triangles?
Answer:
Two geometrical figures having exactly the same shape and size are said to be congruent figures.
Two triangles are congruent to each other if one of them is superimposed on another such that they both cover each other completely.
Two triangles are said to be congruent if the sides and angles of one triangle are exactly equal to the corresponding sides and angles of the other triangle.
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From the figure, △ABC is congruent to △DEF and it is written as △ABC ≌ △DEF
In two congruent triangles, corresponding parts of corresponding angles generally written as ‘c.p.c.t’ are equal they are:
∠ A = ∠ D,
∠ B = ∠ E,
∠ C = ∠ F
and
AB = DE ,
BC = EF ,
AC = DF.
