If two triangles are equal in all respects, they are said to be Congruent triangles. Thus two congruent-triangles have the same shape and same size.
Let ΔABC and ΔPQR be two triangles. Then we can superimpose ΔABC on ΔPQR, so as to cover exactly.
Due to this superimposition :
Vertex A falls on Vertex P
Vertex B falls on Vertex Q
Vertex C falls on Vertex R
AB = PQ ∠A = ∠P
BC =QR ∠B = ∠Q
AC = PR ∠C = ∠R
Hence triangles ABC and PQR are congruent to each other.
Note : 1) Congruent-triangles are similar but the similar triangles are not always congruent.
2) The symbol ≅ reads " is congruent to ".
If two triangles are congruent then there is one to one correspondence (↔) between the two triangles.
ΔABC ↔ ΔPQR then
∠A ≅ ∠P , ∠B ≅ ∠Q and ∠C ≅ ∠R
∠AB ≅ ∠PQ , ∠BC ≅ ∠QR and ∠AC ≅ ∠PR .
Note : If two triangles are congruent then their corresponding parts are congruent.
Corresponding Parts of Congruent Triangles are Congruent ⇒ C. P .C. T. C
1) Every triangle is congruent to itself. ΔABC ≅ ΔABC.
2) If ΔABC ≅ ΔPQR then ΔPQR = ΔABC.
3) If ΔABC ≅ ΔPQR and ΔABC ≅ ΔDEF then ΔPQR ≅ ΔDEF.
In the following pairs of triangles, find out whether the triangles in each pair are congruent or not.
1) ΔABC : AB = 3 , BC = 4 and ∠B = 900
ΔDEF : DE = 3 , DF = 4 and ∠E = 900.
Here , ΔABC not ≅ Δ DEF because there is no one to one correspondence between BC and DF.
2) Δ ABC : AB = 3 , AC = 5 and BC = 6
Δ PQR : PQ = 3 , PR = 5 and QR = 6
Here ΔABC ≅ ΔPQR because there is one to one correspondence between all the sides.