We can add angles and get the result of their sum, and we can also subtract them to find the difference between them.

Even if the angles don't have any numbers, we'll learn how to represent their sum or difference and arrive at the correct result.

We can add angles and get the result of their sum, and we can also subtract them to find the difference between them.

Even if the angles don't have any numbers, we'll learn how to represent their sum or difference and arrive at the correct result.

To find the sum of angles, they must have a common vertex.

Just as we have added angles, we can also subtract one from another.

**We can say that:**

$∡BAE+∡EAC=∡BAC$

Angle A is equal to 30°.

Angle B is equal to 60°.

Angle C is equal to 90°.

Can these angles form a triangle?

Even if angles don't have any numbers, we'll learn how to represent their sum or difference and arrive at the correct result: the correct naming of the angle we get as a result.

Don't worry, the sum and difference of angles is not a difficult topic and mainly relies on the representation of the angles.

Don't know how to correctly mark angles? Go practice representing angles and come back with 90% success!

Let's look at the following example

**We can say that:**

$∡BAE+∡EAC=∡BAC$

It is known that the whole is composed of the sum of its parts, and the same is true with angles.

The large angle A) is made up of the two angles it contains.

If we add the 2 angles that make up angle A), we will obtain this angle.

If we know the size of the angles, we can, with a simple mathematical operation, discover the real value of angle A).

**For example, having the following:**

$∡BAC=30°$

$∡EAC=35°$

and we were asked to calculate: $∡BAC$

which is actually the large angle A) that contains the two given angles inside,

all we have to do is add the values of the given angles and find the one we were asked to discover.

**We can say that:**

$∡BAC=30°+35°=65°$

Just as we have added angles, we can also subtract one angle from another.** Let's look at the following example:**** If we know that:**

$∡BAC=65°$

$∡BAE=30°$

What will be the value of $∡EAC$?

Since angle $∡BAC$ contains the angles $∡BAE$ and $∡EAC$ and is composed only of these two,

we can subtract the given angle $∡BAE$ from the larger angle $∡BAC$ to find the angle $∡EAC$.** That is:**

$∡EAC=65-30=35$

$∡EAC=35°$

Remember: The whole is composed of the sum of its parts!

We can add and subtract angles that are on the same vertex without any problem.

Just pay attention to do it the right way and know how to read the names of the angles.

Test your knowledge

Question 1

Angle A equals 56°.

Angle B equals 89°.

Angle C equals 17°.

Can these angles make a triangle?

Question 2

Angle A equals 90°.

Angle B equals 115°.

Angle C equals 35°.

Can these angles form a triangle?

Question 3

BO bisects \( ∢ABD \).

\( ∢\text{ABD}=85 \)

Calculate the size of

\( \sphericalangle ABO\text{.} \)

**Assignment**

Calculate the value of $X$

**Solution**

We calculate $\sphericalangle ACB$

$\sphericalangle ACB=180-111=69$

Now we calculate $\sphericalangle ABC$

Remember that the sum of all angles in a triangle equals $180^o$

$\sphericalangle ABC=180-69-60=51$

**Answer**

$51$

**Assignment**

Given the angles between parallel lines in the graph, what is the value of: $x$?

**Solution**

$X=?$

$180^o-105^o=75^o$

$75^o+X=110^o$ $/-75^o$

$X=110^o -75^o$

$35^o$

**Answer**

$35^o$

Do you know what the answer is?

Question 1

\( ∢\text{ABD}=15 \)

BD bisects the angle.

Calculate the size of \( ∢\text{ABC} \).

\( \)

Question 2

What type of angle is \( \alpha \)?

\( \)

Question 3

In a right triangle, the sum of the two non-right angles is...?

**Prompt**

Given the parallel lines $a,b$

Find the angle $\alpha$

**Solution**

We extend the vertical line to the end and label the adjacent angles $\beta$ and: $\gamma$ with $\beta$ on the left and: $\gamma$ on the right

Now we notice that the angle $\beta$ is a corresponding angle to: $90^o$ and since adjacent angles sum up to: $180^o$, then the angle $\gamma$ is also equal to: $90^o$

The remaining angle in the small triangle we created, which is also adjacent to: $120^o$ is called $\delta$

As it is adjacent to: $120^o$ it will be equal to: $60^o$ since it is complementary to: $180^o$

Now we calculate the sum of angles in the small triangle:

$180=\alpha+\gamma+\delta$

We replace with the data we know

$180=\alpha+90+60$

$180=\alpha+150$

We move the terms

$\alpha=180-150$

$\alpha=30$

**Answer**

$30$

**Assignment**

$\triangle ABC$ is a triangle

Based on the information, what is the size of the angle $\sphericalangle BAD$

of value $X$?

**Solution**

First, we calculate the angle $\sphericalangle B$

$\sphericalangle B=180-28-86=66$

Now let's find the angle $\sphericalangle ADB$

$\sphericalangle ADB=180-122=58$

Now we refer to the triangle $\triangle ABD$

$\sphericalangle BAD=180-66-58=56$

**Answer**

$56$

Check your understanding

Question 1

The sum of the adjacent angles is 180

Question 2

What is the size of each angle in an equilateral triangle?

Question 3

Calculate the size of angle X given that the triangle is equilateral.

**Assignment**

Calculate the values of $Y$ and $X$

**Solution**

We refer to triangle $\triangle ABC$

Let's find the angle $Y$

$\angle Y=180-47-90=43$

Now we refer to triangle $\triangle ACD$

Let's find the angle $X$

$\angle X=180-90-43=47$

**Answer**

$Y=43, X=47$

Do you think you will be able to solve it?

Question 1

What is the value of the void angle?

Question 2

BE bisects \( ∢\text{FBD} \).

\( ∢\text{FBE}=25 \)

Calculate the size of \( ∢\text{EBD} \).

Question 3

\( ∢DBC=90° \)

BE cross \( ∢\text{DBA} \)

Find the value \( \alpha \)

Angle B is equal to 60°.

Angle C is equal to 90°.

Can these angles form a triangle?

We add the three angles to see if they equal 180 degrees:

$30+60+90=180$

The sum of the angles equals 180, so they can form a triangle.

Yes

Angle A equals 56°.

Angle B equals 89°.

Angle C equals 17°.

Can these angles make a triangle?

We add the three angles to see if they are equal to 180 degrees:

$56+89+17=162$

The sum of the given angles is not equal to 180, so they cannot form a triangle.

No.

Angle A equals 90°.

Angle B equals 115°.

Angle C equals 35°.

Can these angles form a triangle?

We add the three angles to see if they are equal to 180 degrees:

$90+115+35=240$

The sum of the given angles is not equal to 180, so they cannot form a triangle.

No.

Triangle ADE is similar to isosceles triangle ABC.

Angle A is equal to 50°.

Calculate angle D.

Triangle ABC is isosceles, therefore angle B is equal to angle C. We can calculate them since the sum of the angles of a triangle is 180:

$180-50=130$

$130:2=65$

As the triangles are similar, DE is parallel to BC

Angles B and D are corresponding and, therefore, are equal.

B=D=65

$65$°

What is the size of angle ABC?

To calculate the value of angle ABC, we will calculate the sum of all angles included in it.

That is:

$ABC=60+50$

$ABC=110$

110

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- Area
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- Area of a trapezoid
- Perimeter of a trapezoid
- Parallelogram
- The area of a parallelogram: what is it and how is it calculated?
- Perimeter of a Parallelogram
- Kite
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- Corresponding angles
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- Adjacent angles
- Rectangle
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- Right angle
- Acute Angles
- Obtuse Angle
- Plane angle
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- Types of Triangles
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- Identification of an Isosceles Triangle
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- Acute triangle
- Isosceles triangle
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- Area of a Scalene Triangle
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