# JUSTIFYING ANGLE RELATIONSHIPS

Justifying angle relationships :

A transversal is a line that intersects two lines in the same plane at two different points. Transversal and the two lines form eight angles.

In this section, we are going to justify the relationships between the eight angles.

## Justifying angle relationships

Corresponding angles :

Angles lie on the same side of the transversal t, on the same side of lines a and b.

Example : ∠ 1 and ∠ 5

Alternate interior angles :

Angles are nonadjacent angles that lie on opposite sides of the transversal t, between lines a and b.

Example : ∠ 3 and ∠ 6

Alternate exterior angles :

Angles lie on opposite sides of the transversal t, outside lines a and b.

Example : ∠ 1 and ∠ 8

Same-side interior angles :

Angles lie on the same side of the transversal t, between lines a and b.

Example : ∠ 3 and ∠ 5

## Justifying angle relationships - Examples

Example 1 :

A transversal cuts the two parallel lines and forms eight angles.  Describe the relationships between the angles in the diagram given below.

Solution :

Corresponding Angles :

∠CGE and ∠AHG, ∠DGE and ∠BHG, ∠CGH and ∠AHF, ∠DGH and ∠BHF ; congruent.

Alternate interior angles :

∠CGH and ∠BHG, ∠DGH and ∠AHG ; congruent.

Alternate exterior angles :

∠CGE and ∠BHF, ∠DGE and ∠AHF ; congruent.

Same-side interior angles :

∠CGH and ∠AHG, ∠DGH and ∠BHG ; supplementary.

Example 2 :

In the figure given below,  let the lines l₁ and l₂ be parallel and m is transversal. If F = 65°, using the angle relationships,  find the measure of each of the remaining angles.

Solution :

From the given figure,

F and H are vertically opposite angles and they are equal.

Then, H  =  F -------> H  =  65°

H and D are corresponding angles and they are equal.

Then, D  =  H -------> D  =  65°

D and B are vertically opposite angles and they are equal.

Then, B  =  D -------> B  =  65°

F and E are together form a straight angle.

Then, we have

F + E  =  180°

Plug F  =  65°

F + E  =  180°

65° + E  =  180°

E  =  115°

E and G are vertically opposite angles and they are equal.

Then, G  =  E -------> G  =  115°

G and C are corresponding angles and they are equal.

Then, C  =  G -------> C  =  115°

C and A are vertically opposite angles and they are equal.

Then, A  =  C -------> A  =  115°

Therefore,

A  =  C  =  E  =  G  =  115°

B  =  D  =  F  =  H  =  65°

Example 3 :

In the figure given below,  let the lines l₁ and l₂ be parallel and t is transversal. Using angle relationships, find the value of "x"

Solution :

From the given figure,

(2x + 20)° and (3x - 10)° are corresponding angles.

So, they are equal.

Then, we have

2x + 20  =  3x - 10

30  =  x

Hence, x  =  30°

Example 4 :

In the figure given below,  let the lines l₁ and l₂ be parallel and t is transversal. Using angle relationships, find the value of "x"

Solution :

From the given figure,

(3x + 20)° and 2x° are consecutive interior angles.

So, they are supplementary.

Then, we have

3x + 20 + 2x  =  180°

5x + 20  =  180°

5x  =  160°

x  =  32°

Hence, x  =  32°

After having gone through the stuff given above, we hope that the students would have understood "Justifying angle relationships".

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