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include component="navigation" If two sides ( CA and CB ) and the included angle ( BCA ) of a triangle are congruent to the corresponding two sides ( C'A' and C'B' ) and the included angle ( B'C'A' ) in another triangle, then the two triangles are congruent. || Notice the corresponding angles for the two triangles in the applet are the same. The corresponding sides lengths are the same only when the scale factor slider is set at 1.0. Study the side lengths closely and you will find that the corresponding sides are proportional. ==Congruent Triangles== **Congruent triangles** are a special type of similar triangles. Congruent triangles have the same shape (similar triangles) and size. Increase/decrease **<A, <B** and **<C** by clicking and dragging the vertices of the left triangle below. Notice the corresponding angles (**<D** **<E** and **<F**) remain congruent. Since the sum of three angles must be 180o, the third pair of corresponding angles must also be congruent when the first two pairs of corresponding angles are congruent. In the applet above: <A = <D, <B = <E, <C = <F Notice the size and shape of the new pairs of triangles remains the same. The patterns you may have observed and need to know for congruent triangles are displayed below. Remember " ≅ " means "is congruent to". Two triangles are congruent if: || **Corresponding Triangles** a/f = b/d = c/e = **1** || <B = <E <C = <F || AB = DE BC = EF AC = DF ||
 * ===__Side-Angle-Side (SAS) Congruence Postulate__===
 * two pairs of corresponding angles are congruent (therefore the third pair of corresponding angles are also congruent).
 * the three pairs of corresponding sides are proportional.
 * all 6 pairs of corresponding angles and sides are congruent.
 * **Corresponding Congruent Angles** || **Corresponding Congruent Sides**
 * factor** = **1**
 * Δ ABC ≅ Δ DEF || <A = <D

The **factor** for congruent triangles is 1. Remember this fact for future problem solving activities. Move the corners **A**, **B** or **C** of the triangle above. Watch how Δ DEF changes. Since the 3 pairs of corresponding sides of the two triangles are equal each time you manipulate the applet, the **SSS (Side, Side, Side) Congruence Relation** proves each pair of triangles are congruent.
 * If all 3 pairs of corresponding sides in two triangles are the same, then the triangles are congruent.**

__Side-Angle-Side (SAS) Congruence Postulate__
If two sides ( CA and CB ) and the included angle ( BCA ) of a triangle are congruent to the corresponding two sides ( C'A' and C'B' ) and the included angle ( B'C'A' ) in another triangle, then the two triangles are congruent.



**Example 1:** Let ABCD be a parallelogram and AC be one of its diagonals. What can you say about triangles ABC and CDA? Explain your answer.




 * Solution to Example 1:**
 * In a parallelogram, opposite sides are congruent. Hence sides BC and AD are congruent, and also sides AB and CD are congruent.
 * In a parallelogram opposite angles are congruent. Hence angles ABC and CDA are congruent.
 * Two sides and an included angle of triangle ABC are congruent to two corresponding sides and an included angle in triangle CDA. According to the above postulate the two triangles ABC and CDA are congruent.

Congruent Triangles
Triangles that have exactly the same size and shape are called **congruent triangles.** The symbol for congruent is ≅. Two triangles are congruent when the three sides and the three angles of one triangle have the same measurements as three sides and three angles of another triangle. The triangles in Figure [|1] are congruent triangles. 
 * [[image:http://media.wiley.com/Lux/65/18065.nfg023.jpg align="absmiddle"]] ||
 * || **Figure 1** || Congruent triangles. ||  ||
 * [[image:http://media.wiley.com/Lux/65/18065.nfg023.jpg align="absmiddle"]] ||
 * || **Figure 1** || Congruent triangles. ||  ||
 * || **Figure 1** || Congruent triangles. ||  ||
 * || **Figure 1** || Congruent triangles. ||  ||

Congruent Triangles
Three noncolinear points determine a triangle. Draw the three segments connecting the three pairs of points and find three sides, and 3 interior angles, hence the name triangle. Two triangles are congruent if one can be moved on top of the other, so that edges and vertices coincide. The corresponding sides have the same lengths, and corresponding angles are congruent. Assume the edges of one triangle are the same lengths as the corresponding edges of another triangle. Move the first triangle onto the second so that the bases coincide. Do the apexes also coincide? Both apexes are x units away from the left end of the base, and y units from the right end of the base. Since the triangles are oriented the same way, both apexes lie above the base. Draw a circle of radius x centered at the left end of the base, and a circle of radius y centered at the right end of the base. These circles intersect in precisely two points, one above the base and one below. Thus there is only one possible location for the apex. Both apexes coincide, and the first triangle lies directly on top of the second. Corresponding angles coincide, and are congruent. This method of proving triangle congruence is called SSS, for side side side.

Next assume the angle of a triangle, and the adjacent sides that form that angle, are congruent to the angle and adjacent sides of a second triangle. Move the first triangle onto the second so that side angle side lines up with side angle side. If the left sides line up, and the right sides don't, one angle is inside the other, hence smaller than the other. Since the angles are congruent, both pairs of sides line up. Does the vertex at the end of the left side of the first triangle coincide with the vertex at the end of the left side of the second triangle? Both are a fixed distance from the common apex, along a common line, hence they are the same point. By the same reasoning, the third vertex of the first triangle coincides with the third vertex of the second triangle. The sides and angles all coincide and the triangles are congruent. This method of proving congruence is called SAS, for side angle side.

Assume the base and base angles of two triangles are congruent. Place one on top of the other so that the bases coincide. The apex is now the intersection of two lines, at specific angles to the base. These lines are the same in both triangles, and their intersection is the same point, hence both triangles have the same apex, and coincide. This method of proving congruence is called ASA, for angle side angle.

Assume the base, a base angle, and the apex angle of one triangle are congruent to their counterparts in another. Later on we will prove that the angles of a triangle [|add up to 180°]. This means the remaining base angle in the first triangle has the same measure as the remaining base angle in the second. The triangles are congruent by ASA. This method of proving congruence is called SAA, for side angle angle.


 * In general, the method of SSA does not guarantee congruence. Place a 5 12 13 right triangle in the corner of a 9 12 15 right triangle, where 12 is the common side. Let the acute base angle be θ. Subtract the first region from the second and find a triangle with sides 13 4 and 15, and angle θ. Now reflect the smaller right triangle through the common side of length 12. This gives a large triangle with sides 13 15 and 14, and angle θ. These are different triangles, yet they have a side side angle congruence. The problem is, we can flip the segment of length 13 back and forth, creating two different triangles. In general, we have congruence when the second side of SSA (not incident to the angle) is at least as long as the first. Then the aforementioned flip is not possible. ||  || [[image:http://www.mathreference.com/pic/geo,congru-ssa.gif caption="Side Side Angle"]] ||

Assume the leg and hypotenuse of one triangle are equal to the leg and hypotenuse of another. Like SAA above, this relies on a theorem we haven't proved yet, but I wanted to keep all the triangle correspondence results together. The [|Pythagorean theorem] tells us the remaining legs in the two triangles are equal. Hence they are congruent by SSS. This method of proving congruence is called LH, for leg hypotenuse. || include component="navigation" **Congruent triangles** are a special type of similar triangles. Congruent triangles have the same shape (similar triangles) and size. Increase/decrease **<A, <B** and **<C** by clicking and dragging the vertices of the left triangle below. Notice the corresponding angles (**<D** **<E** and **<F**) remain congruent. Since the sum of three angles must be 180o, the third pair of corresponding angles must also be congruent when the first two pairs of corresponding angles are congruent. Two [|triangles] are congruent if their corresponding [|sides] are equal in length and their corresponding [|angles] are equal in size. If triangle ABC is congruent to triangle DEF, the relationship can be written mathematically as: In many cases it is sufficient to establish the equality of three corresponding parts and use one of the following results to deduce the congruence of the two triangles. Congruent triangles can be rotated and/or mirror images of each other (reflected). (See [|Congruent triangles].) In the figure on the right, the two triangles have all three corresponding sides equal in length and so are still congruent, even though one is the mirror image of the other and rotated.
 * A right triangle contains a right angle, whence the other two angles are acute and complementary. The side opposite the right angle is the hypotenuse. The sides adjacent to the right angle are called the legs. We usually orient the triangle so that one of the legs is horizontal, the base, and the other is vertical, the altitude.