Calculate C¹B¹ in a Rectangular Prism with Diagonal √(6x²-12x+41)

Rectangular Prism Diagonals with Algebraic Expressions

ABCDA1B1C1D1 ABCDA^1B^1C^1D^1 is a rectangular prism.

The length of its diagonal is

6x212x+41 \sqrt{6x^2-12x+41} .


DC1=5x24x+25 DC^1=\sqrt{5x^2-4x+25}

Calculate C1B1 C^1B^1 .

AAABBBCCCDDDAAA111BBB111CCC111DDD111

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:16 First, let's calculate C one B one.
00:25 Each face of the box is a rectangle, so every angle is a right angle.
00:34 Now, use the Pythagorean theorem in triangle D B one C one to find C one B one.
00:45 Substitute the given values and solve for C one B one.
01:06 Next, isolate C one B one and simplify the terms.
01:23 And that's how we solve this problem.

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

ABCDA1B1C1D1 ABCDA^1B^1C^1D^1 is a rectangular prism.

The length of its diagonal is

6x212x+41 \sqrt{6x^2-12x+41} .


DC1=5x24x+25 DC^1=\sqrt{5x^2-4x+25}

Calculate C1B1 C^1B^1 .

AAABBBCCCDDDAAA111BBB111CCC111DDD111

2

Step-by-step solution

To solve this problem, we'll use the expressions for the diagonals given:

  • The space diagonal of the prism: 6x212x+41 \sqrt{6x^2 - 12x + 41} .
  • The face diagonal DC1=5x24x+25 DC^1 = \sqrt{5x^2 - 4x + 25} .
  • We need to find the edge C1B1 C^1B^1 .

Since DC1=a2+b2 DC^1 = \sqrt{a^2 + b^2} , we will denote one dimension as aa and another as bb. Therefore:

DC1=5x24x+25=(x2)2+(b)2 DC^1 = \sqrt{5x^2 - 4x + 25} = \sqrt{(x-2)^2 + (b)^2} .

Assuming that x2x-2 is one of the lengths, we have:

5x24x+25=(x2)2+b2 5x^2 - 4x + 25 = (x-2)^2 + b^2 .

On expanding (x2)2(x-2)^2, we get x24x+4 x^2 - 4x + 4 . Thus:

5x24x+25=x24x+4+b2 5x^2 - 4x + 25 = x^2 - 4x + 4 + b^2 .

Cancelling the x24xx^2 - 4x terms on both sides gives,

4x2+21=b2 4x^2 + 21 = b^2 .

For the diagonal of the prism, we assume if x2 x-2 is one dimension and b=4x2+21 b = \sqrt{4x^2 + 21} , we solve:

(6x212x+41)2=(x2)2+(b)2+(C1B1)2 \left(\sqrt{6x^2 - 12x + 41}\right)^2 = (x-2)^2 + (b)^2 + ( C^1B^1 )^2 .

6x212x+41=(x2)2+4x2+21+(C1B1)2 6x^2 - 12x + 41 = (x-2)^2 + 4x^2 + 21 + (C^1B^1)^2 .

Substituting (x2)2=x24x+4 ( x-2 )^2 = x^2 - 4x + 4 :

6x212x+41=x24x+4+4x2+21+(C1B1)2 6x^2 - 12x + 41 = x^2 - 4x + 4 + 4x^2 + 21 + (C^1B^1)^2 .

Simplify and solve for C1B1 C^1B^1 :

6x212x+41=5x2+25+(C1B1)2 6x^2 - 12x + 41 = 5x^2 + 25 + (C^1B^1)^2.

Thus, x212x+16=(C1B1)2x^2 - 12x + 16 = (C^1B^1)^2.

This implies C1B1=x4C^1B^1 = x - 4.

Therefore, the solution to the problem is C1B1=x4 C^1B^1 = x-4 .

3

Final Answer

x4 x-4

Key Points to Remember

Essential concepts to master this topic
  • Space Diagonal Formula: d=a2+b2+c2 d = \sqrt{a^2 + b^2 + c^2} for dimensions a, b, c
  • Face Diagonal Method: Use DC1=a2+b2 DC^1 = \sqrt{a^2 + b^2} to find two dimensions first
  • Verification: Substitute back: (x4)2+(x2)2+(4x2+21)2=6x212x+41 (x-4)^2 + (x-2)^2 + (\sqrt{4x^2+21})^2 = 6x^2-12x+41

Common Mistakes

Avoid these frequent errors
  • Confusing face diagonal with space diagonal formulas
    Don't use the space diagonal formula a2+b2+c2 \sqrt{a^2 + b^2 + c^2} for face diagonals = wrong dimensions! Face diagonals only involve two dimensions, not three. Always identify whether you're working with a face diagonal (2D) or space diagonal (3D) first.

Practice Quiz

Test your knowledge with interactive questions

Look at the triangle in the diagram. How long is side AB?

222333AAABBBCCC

FAQ

Everything you need to know about this question

What's the difference between a space diagonal and face diagonal?

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A space diagonal goes through the interior of the prism from one corner to the opposite corner, involving all three dimensions. A face diagonal lies on one face of the prism, only involving two dimensions.

How do I know which dimensions to use for each diagonal?

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Look at the vertices! DC1 DC^1 connects D to C¹, so it's a face diagonal on the rectangular face. The space diagonal connects opposite corners like AC1 AC^1 .

Why did we assume x-2 is one of the dimensions?

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We factored the expression under the square root. Since 5x24x+25 \sqrt{5x^2-4x+25} contains perfect square patterns, we tried (x2)2 (x-2)^2 and it worked!

What if I can't factor the expressions neatly?

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Sometimes you need to complete the square or use algebraic manipulation. Look for patterns like a2±2ab+b2 a^2 ± 2ab + b^2 that suggest perfect squares.

How do I check my final answer?

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Substitute C1B1=x4 C^1B^1 = x-4 back into the space diagonal formula. All three dimensions should give you 6x212x+41 \sqrt{6x^2-12x+41} when combined properly.

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