What is vertically opposite angles?
When two straight lines intersect each other four angles are formed.
The pair of angles which lie on the opposite sides of the point of intersection are called vertically opposite angles.
In the given figure, two straight lines AB and CD intersect each other at point O. Angles AOD and BOC form one pair of vertically opposite angles; whereas angles AOC and BOD form another pair of vertically opposite angles.
Vertically opposite angles are always equal.
i.e., ∠AOD = ∠BOC
and ∠AOC = ∠BOD
Note:
In the given figure; rays OM and ON meet at O to form ∠MON (i.e. ∠a) and reflex ∠MON (i.e. ∠b). It must be noted that ∠MON means the smaller angle only unless it is mentioned to take otherwise.
For example; in the given figure, two lines WX and YZ are intersecting at a point O.
We observe that with the intersection of these lines, four angles have been formed. Angles ∠1 and ∠3 form a pair of vertically opposite angles; while angles ∠2 and ∠4 form another pair of vertically opposite angles.
Clearly, angles ∠1 and ∠2 form a linear pair.
Therefore, ∠1 + ∠2 = 180°
or, ∠1 = 180°  ∠2 …………(i)
again similarly, ∠2 and ∠3 form a linear pair.
Therefore, ∠2 + ∠3 = 180°
or, ∠3 = 180°  ∠2 …………(ii)
From (i) and (ii) we get;
∠1 = ∠3
Similarly, we can prove that ∠2 = ∠4
Thus, if two lines intersect then vertically opposite angles are always equal.
In the below figure, ∠1 and ∠2 are not vertically opposite angles, because their arms do not form two pairs of opposite rays.
Now, we will solve various examples on vertically opposite angles.
Workedout problems on vertically opposite angles:
1. In the given figure, find the measure of unknown angles.
Solution:
(i) ∠3= 60° vertically opposite angles
(ii) ∠2 = 90° vertically opposite angles
(iii) ∠2 + ∠1 + 60° = 180° (straight angle)
90° + ∠ 1 + 60° = 180°
150° + ∠ 1 = 180°
Therefore, ∠1 = 180° — 150° = 30°
(iv) ∠1 = ∠4 vertically opposite angles
Therefore, ∠4 = 30°
2. In the given figure, lines PQ, RS, TV intersect at O. If x : y : z = 1 : 2 : 3, then find the values of x, y, z.
Solution:
The sum of all the angles at a point is 360°.
∠POR = ∠SOQ = x° (Pair of vertically opposite angles are equal.)
∠VOQ = ∠POT = y° (Pair of vertically opposite angles are equal.)
∠TOS = ∠ROV = z° (Pair of vertically opposite angles are equal.)
Therefore, ∠POT + ∠POR + ∠ROV + ∠VOQ + ∠QOS + ∠SOT = 360°
y + x + z + y + x + z = 360°
⟹ 2x + 2y + 2z = 360°
⟹ 2(x + y + z) = 360°
⟹ x + y + z = 3̶6̶0̶°/2̶
⟹ x + y + z = 180°  (i)
Let the common ratio be a.
Therefore, x = a, y = 2a, z = 3a
Therefore, from the equation (i) we get;
a + 2a + 3a = 180°
⟹ 6a = 180°
⟹ a = 1̶8̶0̶°/6̶
⟹ a = 30°
Therefore, x = a, means x = 30°
y = 2a, means y = 2 × 30 = 60°
z = 3a, means z = 3 × 30 = 90°
Therefore, the measures of the angles are 30°, 60°, 90°.
● Lines and Angles
Fundamental Geometrical Concepts
Some Geometric Terms and Results
Complementary and Supplementary Angles
Parallel and Transversal Lines
7th Grade Math Problems
8th Grade Math Practice
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