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Corresponding Sides
Corresponding sides of a polygon are the sides that are in the same position in similar polygons. In geometry, finding the congruence and similarity involves comparing corresponding sides and corresponding angles of the polygons. In this article, let's learn more about similar right triangles, corresponding sides, their definition, how they are proportional, the differences between congruent and similar triangles with a few solved examples.
Definition of Corresponding Sides
Corresponding sides are the sides that are in the same position in any different 2dimensional shapes. For any two polygons to be congruent, they must have exactly the same shape and size. This means that all their interior angles and their corresponding sides must be the same measure. For any two polygons to be similar, the ratios of the lengths of each pair of corresponding sides must be equal. Let us consider 2 quadrilaterals ABCD and PQRS to understand the corresponding sides.
From the above image, we can observe that:
 The side AB corresponds to the side PQ
 The side BC corresponds to the side QR
 The side CD corresponds to the side RS
 The side DA corresponds to the side SP
The SSS  Congruence rule states that, in two triangles, if all the three sides of one triangle are equivalent to the corresponding three sides of the second triangle, then the two triangles can be considered to be congruent. In a triangle, the corresponding sides are the sides that are in the same position in different triangles. In the belowgiven images, the two triangles are congruent and their corresponding sides are colorcoded.
In the above two triangles ABC and XYZ,
 AB is the corresponding side to XY
 BC is the corresponding side to YZ
 CA is the corresponding side to ZX
Congruent Triangles vs Similar Triangles
The congruent triangles are different from similar triangles considering the aspect of corresponding sides. The table below shows the differences between congruent and similar triangles with the help of the illustration.
Congruent Triangles  Similar Triangles 

Two triangles are considered to be congruent if all their corresponding angles and sides are equal  Two triangles are considered to be similar if all their corresponding angles are equal and their corresponding sides are in the same ratio 
In (i) Δ ABC and Δ LMN, (1) AB = LM, BC = MN, and AC = LN. (2) ∠A = ∠M, ∠B = ∠L, ∠C = ∠N ΔABC is congruent to Δ LMN 
In (ii) Δ PQR and Δ STU, (1)PQ \(\propto\) ST, QR \(\propto\) TU, and PR \(\propto\) SU (2) ∠P = ∠S, ∠Q = ∠T, ∠R = ∠U Δ PQR is similar to Δ STU 
Corresponding Sides in Similar Triangles
If the two shapes are similar, then their corresponding sides are proportional. In two similar triangles, the corresponding sides are proportional and these corresponding sides always touch the same two angle pairs. In the given similar triangles PQR and STU:
 PQ is the corresponding side to ST, and while PQ touches ∠P and ∠Q, ST touches ∠S and ∠T
 PR is the corresponding side to SU, and while PR touches ∠P and ∠R, SU touches ∠S and ∠U
 QR is the corresponding side to TU, and while QR touches ∠Q and ∠R, TU touches ∠T and ∠U
To understand proportionality, consider a) \(\triangle \text{ABC} \simeq \triangle \text{ADE}\)
AB/AD = AC/AE
AB × AE = AD × AC
Consider b) \(\triangle \text{PQR} \simeq \triangle \text{STU}\)
PQ/ST = PR/SU = QR/TU
Hence, if two triangles are similar, then their corresponding sides are proportional.
Consider two similar triangles ABC and DEF,
In the above image,
AB/DE = BC/EF
10/16 = 9/a
10 × a = 16 × 9
a = (16 × 9)/10
a = 144/10 = 14.4
Thus we conclude that if \(\triangle \text{ABC} \simeq \text{DEF}\), then we say that the corresponding sides are proportional and the angles are equal.
AB/DE = BC/EF = CA/FD = k, where k is the equivalent ratio or the trigonometric ratio.
Corresponding Sides in Right Triangles
If the lengths of the hypotenuse and a leg of one rightangled triangle are proportional to the corresponding parts of the other right triangle, then the triangles are similar. Consider the two right triangles ABC and DEF in the belowgiven image,
\[\dfrac{\text{The shortest side of the small triangle}}{\text{The shortest side of the large triangle}}\\=\dfrac {\text{The longest side of the small triangle}} {\text{The longest side of the large triangle}}\\= \dfrac{\text{Hypotenuse of small triangle}}{\text{Hypotenuse of the large triangle}}\]
a/d = b/e = c/f
Related Articles on Corresponding Sides
Check out the following pages related to the corresponding sides.
Important Notes
Here is a list of a few points that should be remembered while studying corresponding sides:
 When two triangles are similar, the ratios of the lengths of their corresponding sides are equal.
 Two triangles are considered to be congruent if all their corresponding angles and sides are equal
 The SSS  Congruence rule states that, in two triangles, if all the three sides of one triangle are equivalent to the corresponding three sides of the second triangle, then the two triangles can be considered to be congruent.
Solved Examples

Example 1. Brandon wants to check if the two triangles are similar. Can you help him?
Solution:
To determine if the triangles are similar we need to check if the sides are proportional.
Identify the corresponding sides in Δ LMN and Δ XYZ.
\(\dfrac{LN}{XY} = \dfrac{30}{40} = \dfrac{30\div 10}{40\div 10} = \dfrac{3}{4}\)
\(\dfrac{LM}{YZ} = \dfrac{42}{56}= \dfrac{42\div 14}{56\div 14} = \dfrac{3}{4}\)
\(\dfrac{MN}{ZX} = \dfrac{54}{72} = \dfrac{54\div 9}{72\div 9} = \dfrac{3}{4}\)
We find that the corresponding sides are proportional to each other. Therefore, the two triangles LMN and XYZ are similar

Example 2. Micky was arranging a few 2dimensional shapes while she was constructing a math puzzle. She found 2 triangles arranged in the way shown below. On observation, she found that the triangles are similar. She wanted to know which are the corresponding sides of these triangles. How can you help her?
Solution:
AOB and POQ are two similar triangles. If the triangles are similar, then their sides are proportional.
Let us check which two sides form an equal proportion.
\(\dfrac{3}{6}=\dfrac{1}{2}\\\dfrac{4}{8}=\dfrac{1}{2}\\\dfrac{5}{10}=\dfrac{1}{2}\)
\(\dfrac{OA}{OP}=\dfrac{OB}{OQ}=\dfrac{AB}{PQ}=\dfrac{1}{2}\)
From the above ratio, we find out that OA corresponds to OP, OB corresponds to OQ and AB corresponds to PQ.
FAQs on Corresponding Sides
Give an Example of Corresponding Sides.
Consider this example: if one polygon has sequential sides p,q, and r, and the other has sequential sides a,b, and c, and if q and b are corresponding sides, then side p (adjacent to q) must correspond to either a or c (both adjacent to b).
Define Corresponding Sides and Angles.
Sides and angles can be considered as corresponding when a pair of matching angles or sides are in the same position in two different shapes.
What Are the Corresponding Parts of Congruent Triangles?
In two congruent triangles, the sides and angles are considered to be their corresponding parts. The corresponding parts are found in the same relative positions.
What Is the Difference Between Corresponding and Alternate Angles?
Comparing the two angles in 2 similar polygons, the corresponding angles relatively occupy the same position. When a transversal meets two parallel lines, corresponding angles that lie relatively in the same position are considered to be congruent, they are of the same measure. Angles are considered to be alternate angles when they are on the opposite sides of the transversal lines.
What Letter of the Alphabet Has Corresponding Angles?
The letter F is identified to get corresponding angles. The corresponding angles are relatively in the same position when a transversal intersects two parallel lines and they are equal.
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