Calculate the CAD Angle in an Isosceles Triangle with Height AD

Isosceles Triangle Angle Calculations with Height Properties

ABC is an isosceles triangle.

AD is its height.

Calculate the size of angle CAD ∢\text{CAD} .

AAABBBCCCDDD55

❤️ Continue Your Math Journey!

We have hundreds of course questions with personalized recommendations + Account 100% premium

Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Determine angle CAD
00:03 The following is an isosceles triangle according to the given data
00:07 In an isosceles triangle, base angles are equal
00:10 The sum of angles in the triangle (ADC) equals 180
00:22 Isolate CAD
00:34 This is the solution

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

ABC is an isosceles triangle.

AD is its height.

Calculate the size of angle CAD ∢\text{CAD} .

AAABBBCCCDDD55

2

Step-by-step solution

To solve this problem, we'll apply the properties of isosceles and right triangles.

  • Step 1: Recognize that since ABC \triangle ABC is isosceles, angles BAD=CAD \angle \text{BAD} = \angle \text{CAD} , and we are given ABD=55 \angle \text{ABD} = 55^\circ .
  • Step 2: In ABD \triangle ABD , use the angle sum property for a triangle, BAD+ABD+BDA=180 \angle \text{BAD} + \angle \text{ABD} + \angle \text{BDA} = 180^\circ .
  • Step 3: Given that BDA=90 \angle \text{BDA} = 90^\circ (since AD AD is the height), calculate BAD \angle \text{BAD} .

Applying the angle sum property:
BAD+55+90=180 \angle \text{BAD} + 55^\circ + 90^\circ = 180^\circ

Simplifying, we find:
BAD=180145 \angle \text{BAD} = 180^\circ - 145^\circ
BAD=35 \angle \text{BAD} = 35^\circ

Thus, the angle CAD \angle \text{CAD} is also 35 35^\circ because ABC \triangle ABC is isosceles and BAD=CAD \angle \text{BAD} = \angle \text{CAD} .

Therefore, the solution to this problem is 35 35 .

3

Final Answer

35

Key Points to Remember

Essential concepts to master this topic
  • Property: Height from apex creates two congruent right triangles
  • Technique: Use angle sum in right triangle: 35°+55°+90°=180° 35° + 55° + 90° = 180°
  • Check: Both base angles must equal each other in isosceles triangle ✓

Common Mistakes

Avoid these frequent errors
  • Assuming the given 55° is angle CAD
    Don't assume the marked angle is the one you're looking for = wrong starting point! The diagram shows angle ABD = 55°, not angle CAD. Always identify which angle is given by carefully reading labels and diagram markings.

Practice Quiz

Test your knowledge with interactive questions

Is DE side in one of the triangles?
AAABBBCCCDDDEEE

FAQ

Everything you need to know about this question

How do I know which angle is 55° in the diagram?

+

Look carefully at the angle marking in the diagram. The purple arc shows angle ABD = 55°, which is at vertex B, not the angle CAD we need to find.

Why are angles BAD and CAD equal in an isosceles triangle?

+

In an isosceles triangle, the height from the apex (top vertex) bisects the apex angle. This means AD splits angle BAC into two equal parts: angle BAD = angle CAD.

What makes angle BDA equal to 90°?

+

AD is the height of the triangle, which means it's perpendicular to the base BC. A perpendicular line always creates a 90° angle.

Can I solve this without using the angle sum property?

+

The angle sum property (angles in triangle=180° \text{angles in triangle} = 180° ) is the most reliable method. Other approaches might work but are more complex and error-prone.

What if the triangle wasn't isosceles?

+

Without the isosceles property, we couldn't conclude that angle BAD = angle CAD. The height would still create right angles, but we'd need more information to find specific angle measures.

How do I remember which angles are equal in isosceles triangles?

+

Remember: In isosceles triangles, base angles are equal and the height from apex bisects the top angle. Draw it out to visualize these relationships!

🌟 Unlock Your Math Potential

Get unlimited access to all 18 Triangle questions, detailed video solutions, and personalized progress tracking.

📹

Unlimited Video Solutions

Step-by-step explanations for every problem

📊

Progress Analytics

Track your mastery across all topics

🚫

Ad-Free Learning

Focus on math without distractions

No credit card required • Cancel anytime

More Questions

Click on any question to see the complete solution with step-by-step explanations