Finding Length DB in a Complex Triangle Configuration: 6-Unit Height Problem

Pythagorean Theorem with Multiple Applications

Given the triangles in the drawing

What is the length of the side DB?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find DB
00:03 We'll use the Pythagorean theorem in triangle ABC
00:12 We'll substitute appropriate values and solve for BC
00:22 Isolate BC
00:30 This is the length of side BC
00:33 Now we'll use the Pythagorean theorem in triangle BCD
00:43 We'll substitute appropriate values and solve for BD
00:49 Isolate BD
00:56 And this is the solution to the problem

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Given the triangles in the drawing

What is the length of the side DB?

666222AAABBBCCCDDD

2

Step-by-step solution

In this question, we will have to use the Pythagorean theorem twice.

A²+B²=C²

Let's start by finding side CB:

6²+CB²=(2√11)²

36+CB²=4*11

CB²=44-36

CB²=8

CB=√8

We will use the exact same way to find side DB:

2²+DB²=(√8)²

4+CB²=8

CB²=8-4

CB²=4

CB=√4=2

3

Final Answer

2 cm

Key Points to Remember

Essential concepts to master this topic
  • Rule: Apply Pythagorean theorem twice for multi-step problems
  • Technique: Find CB first: 62+CB2=(211)2 6^2 + CB^2 = (2\sqrt{11})^2 , then solve for DB
  • Check: Verify DB = 2 by substituting: 22+22=(8)2 2^2 + 2^2 = (\sqrt{8})^2

Common Mistakes

Avoid these frequent errors
  • Solving for DB without first finding CB
    Don't try to find DB directly from the given measurements = missing crucial information! You need CB as an intermediate step because it forms the hypotenuse of the second triangle. Always identify which triangle to solve first and work systematically through each step.

Practice Quiz

Test your knowledge with interactive questions

Consider a right-angled triangle, AB = 8 cm and AC = 6 cm.
Calculate the length of side BC.

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FAQ

Everything you need to know about this question

Why do I need to use the Pythagorean theorem twice?

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This problem involves two connected right triangles. First, use triangle ABC to find CB, then use triangle CDB to find DB. Each triangle requires its own application of a2+b2=c2 a^2 + b^2 = c^2 .

How do I know which side is the hypotenuse?

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The hypotenuse is always the longest side and sits opposite the right angle. In the diagram, look for the right angle symbols (small squares) to identify which side is opposite to them.

What does 211 2\sqrt{11} equal as a decimal?

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You don't need to convert to decimals! Keep exact values like 211 2\sqrt{11} throughout your work. When you square it: (211)2=4×11=44 (2\sqrt{11})^2 = 4 \times 11 = 44 .

Why is CB = 8 \sqrt{8} and not simplified further?

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Great observation! 8=4×2=22 \sqrt{8} = \sqrt{4 \times 2} = 2\sqrt{2} , but for this problem, leaving it as 8 \sqrt{8} works perfectly fine since we square it in the next step.

How can I check that DB = 2 is correct?

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Substitute back into triangle CDB: CD2+DB2=CB2 CD^2 + DB^2 = CB^2 becomes 22+22=(8)2 2^2 + 2^2 = (\sqrt{8})^2 , which gives 4+4=8 4 + 4 = 8

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