Understanding cot2(theta) = A-C / B

  • Thread starter Terrell
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In summary, the conversation discusses the derivation of an equation involving coefficients A, B, C, and theta, and how the author uses the symbol B' to represent the coefficient of x'y' for simplicity. The process of deducing the third equation involves keeping track of terms and simplifying.
  • #1
Terrell
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Homework Statement


How did it go from A'x'^2 + B'x'y' + C'y'^2 + D'x' +E'y' + F = 0 to become B' = 2(C-A)sin(theta)cos(theta) + B(cos^2(theta) - sin^2(theta))?

*this is not an assignment, I am just self studying :P
 

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  • #2
Expand the top most equation then collect the terms based on the powers of ##x'## and ##y'##. To obtain (12), just keep track on the terms in the top most equation which contain ##x'y'##.
 
  • #3
Terrell said:

Homework Statement


How did it go from A'x'^2 + Bx'y' + C'y'^2 + D'x' +E'y' + F = 0 to become B' = 2(C-A)sin(theta)cos(theta) + B(cos^2(theta) - sin^2(theta))?
The latter is not a deduction from the former. It is simply a definition, which the author is free to make, as the symbol B' has not been used up to that point.

The deduction is the equation immediately after the 'where...' part.
 
  • #4
blue_leaf77 said:
Expand the top most equation then collect the terms based on the powers of ##x'## and ##y'##. To obtain (12), just keep track on the terms in the top most equation which contain ##x'y'##.
yes. i got that part, but how do i get from the 2nd equation to the third equation?
 
  • #5
andrewkirk said:
The latter is not a deduction from the former. It is simply a definition, which the author is free to make, as the symbol B' has not been used up to that point.

The deduction is the equation immediately after the 'where...' part.
but how do i deduce it? what is step1? i don't know how to start.. from A'x'^2 + B'x'y' + ... + F = 0 to B' = 2(C - A)...
 
  • #6
Like andrew has said, ##B'## is just a symbol used to represent the coefficient of ##x'y'##. The author could have retained the original form of the coefficient in terms of ##A##, ##B##, ##C##, and ##\theta##, but it will require more space. He chose ##B'## as the new coefficient for ##x'y'## because this symbol has not been used in the previous derivation.
 
  • #7
i got it! just keep track and combine all the terms with x'y' then equate to zero and simplify. thanks guys!
 
  • #8
can I get the name of the book?
 
  • #9
mkeaudric said:
can I get the name of the book?
This thread is six years old. Don't count on a response from the participants.
 
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1. What is cotangent?

Cotangent is a trigonometric function that represents the ratio of the adjacent side to the opposite side of a right triangle.

2. How is cot2(theta) related to the Pythagorean theorem?

The Pythagorean theorem states that in a right triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the other two sides. In this case, cot2(theta) can be written as (A-C)/B, which follows the same format as the Pythagorean theorem (c^2 = a^2 + b^2).

3. What does A, B, and C represent in cot2(theta) = A-C / B?

In this equation, A, B, and C represent the lengths of the sides of a right triangle, with A being the adjacent side, B being the opposite side, and C being the hypotenuse.

4. How do you solve for theta in cot2(theta) = A-C / B?

To solve for theta, you would use inverse trigonometric functions. In this case, you would take the inverse cotangent (arccot) of both sides of the equation. This would give you theta = arccot((A-C)/B).

5. Can cot2(theta) be negative?

Yes, cot2(theta) can be negative. Just like with any other trigonometric function, the value of cot2(theta) can be positive or negative depending on the position of the angle theta in the coordinate plane.

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