# Justify your answer | Can you find the angle X? | #math #maths | #geometry

TLDRIn this geometry lesson, the presenter tackles the problem of finding angle X in triangle ABC with given angles and side lengths. Utilizing the exterior angle theorem and properties of a 30-60-90 triangle, the video demonstrates step-by-step calculations leading to the conclusion that angle X equals 75°. The explanation involves constructing auxiliary lines and identifying isosceles triangles, ultimately solving the problem with a clear and engaging approach.

### Takeaways

- 📚 The video discusses a geometry problem involving a triangle ABC with a line CD.
- 🔍 Angle ADC is given as 60° and angle DBC as 45°, with angle CAD represented by X.
- 📐 Segment AD is 2 units and segment BD is 1 unit, which are key measurements for solving the problem.
- 📈 The video introduces the Exterior Angle Theorem, stating that an exterior angle of a triangle equals the sum of the two opposite interior angles.
- 🧩 The problem involves a 30-60-90 special triangle, ABC, with specific side ratios.
- 📉 In a 30-60-90 triangle, the hypotenuse is twice the length of the shorter leg, and the longer leg is √3 times the shorter leg.
- ✅ The Triangle Sum Theorem is used to establish that the sum of the interior angles in a triangle is 180°.
- 📐 By applying the theorems, the video deduces that angle X must be 15° by considering the exterior angle of triangle ADC.
- 📏 The video uses auxiliary lines and geometric properties to further analyze the problem and find the lengths of segments AE and ED.
- 🔺 Triangle AED is identified as a 30-60-90 triangle, allowing for the calculation of segment lengths based on the angles.
- 🔄 Triangle BDE is found to be isosceles, leading to the conclusion that angle BDE is 30°.
- 🔄 Triangle AEB is also isosceles, with angles AEB and ABE both being 30°, leading to the determination of other angles in the triangle.
- 🔢 The final step involves adding angles to find angle X, which is the sum of 30° and 45°, resulting in angle X being 75°.

### Q & A

### What is the Exterior Angle Theorem mentioned in the video?

-The Exterior Angle Theorem states that the exterior angle of a triangle is equal to the sum of the two opposite interior angles.

### What is a 30-60-90 triangle and what are its properties?

-A 30-60-90 triangle is a special right triangle where the angles are 30 degrees, 60 degrees, and 90 degrees. The side opposite the 30-degree angle is the shortest, the side opposite the 60-degree angle is the middle length, and the side opposite the 90-degree angle, the hypotenuse, is twice the length of the shortest side.

### According to the Triangle Sum Theorem, what is the sum of the interior angles of a triangle?

-The Triangle Sum Theorem states that the sum of the interior angles of a triangle is always 180 degrees.

### What is the relationship between the sides of a 30-60-90 triangle?

-In a 30-60-90 triangle, the longest leg (opposite the 60-degree angle) is twice the length of the shortest leg (opposite the 30-degree angle), and the middle leg (opposite the 90-degree angle) is the shortest leg multiplied by the square root of 3.

### What is the value of angle X in the given triangle ABC?

-Angle X in the given triangle ABC is 75 degrees, which is the sum of 30 degrees and 45 degrees.

### How is the length of segment AE determined in the video?

-The length of segment AE is determined by recognizing that triangle AED is a 30-60-90 triangle, where AE is opposite the 30-degree angle and AD is the hypotenuse. Since AD is 2 units, AE must be 1 unit.

### Why is triangle BDE considered isosceles?

-Triangle BDE is considered isosceles because the side lengths ED and BD are equal, making the angles opposite those sides congruent, which includes the 30-degree angle at B.

### How does the video establish that triangle AEB is isosceles?

-The video establishes that triangle AEB is isosceles by showing that angles AEB and ABE are both 30 degrees, making AE equal to AB.

### What is the significance of the 45-degree angle in triangle BEC?

-The 45-degree angle in triangle BEC, along with the other 15-degree angles, indicates that triangle BEC is isosceles, with BE equal to CE.

### How is the final angle X calculated in the video?

-The final angle X is calculated by adding the two individual angles that form angle X, which are 30 degrees and 45 degrees, resulting in 75 degrees.

### Outlines

### 📚 Introduction to Triangle Geometry Problem

The video begins with an introduction to a geometry problem involving triangle ABC and line CD. The angles ADC and BDC are given as 60° and 45°, respectively, and angle CAD is represented by variable X. The lengths of segments AD and BD are provided as 2 units and 1 unit, respectively. The task is to calculate the value of angle X. The presenter reminds viewers to subscribe and notes that the diagram may not perfectly represent the problem. The video then reviews the exterior angle theorem, which states that an exterior angle of a triangle is equal to the sum of the two opposite interior angles. The right triangle ABC is identified as a 30-60-90 special triangle, with the side lengths following a specific ratio. The Triangle Sum Theorem is also discussed, which states that the sum of the interior angles in a triangle is always 180°.

### 🔍 Detailed Analysis and Solution of the Geometry Problem

The second paragraph delves into a detailed analysis of the problem. The presenter uses the exterior angle theorem to deduce that the unknown angle must be 15°, as it is the sum of two other given angles. By applying the straight angle property, which is 180°, the presenter determines another angle to be 120°. The presenter then introduces auxiliary lines and focuses on right triangle AED, identifying it as a 30-60-90 triangle and using its properties to find the lengths of the sides. The connection between points E and B is made, revealing that triangle BDE is isosceles, with congruent angles of 30°. The presenter continues to analyze the larger triangle AEB, concluding it is also isosceles with sides of equal length. The final step involves focusing on triangle BEC, identifying it as an isosceles triangle with angles of 15° each. The presenter then concludes that triangle AEC is isosceles as well, with angles of 45°. The final calculation for angle X is presented as the sum of two angles, 30° and 45°, resulting in a final answer of 75° for angle X. The video concludes with a reminder to subscribe for more educational content.

### Mindmap

### Keywords

### 💡Exterior Angle Theorem

### 💡30-60-90 Triangle

### 💡Hypotenuse

### 💡Triangle Sum Theorem

### 💡Perpendicular

### 💡Isosceles Triangle

### 💡Angle Sum of a Triangle

### 💡Auxiliary Lines

### 💡Special Triangle

### 💡Right Angle

### 💡Straight Angle

### Highlights

Introduction to the problem involving triangle ABC with a line CD and angles ADC, BDC, and CAD represented by X.

Explanation of the exterior angle theorem relating to the sum of opposite interior angles.

Identification of triangle ABC as a 30-60-90 special triangle with specific angle and side relationships.

Description of the properties of a 30-60-90 triangle, including the hypotenuse being twice the shortest leg.

Application of the Triangle Sum Theorem to find the third angle in triangle ABC.

Use of the exterior angle theorem to deduce the unknown angle to be 15°.

Introduction of the straight angle property to find the remaining angle in the figure.

Drawing auxiliary lines and focusing on right triangle AED to deduce its properties.

Identification of triangle AED as a 30-60-90 triangle and calculating the lengths of its sides.

Connecting points E and B to form isosceles triangle BDE and deducing its angles.

Observation of angles in triangle AEB leading to the conclusion that it is isosceles.

Calculation of the remaining angle in triangle BEC as 15° using the properties of isosceles triangles.

Focus on triangle AEC and its properties to deduce the angles as 45°.

Final calculation of angle X as the sum of two individual angles, resulting in 75°.

Conclusion of the video with the final answer for angle X and a call to action for subscriptions.