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

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

666888BBBCCCAAA

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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