Find angle to eliminate cross product term

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SUMMARY

The discussion focuses on eliminating the cross product term in the quadratic equation 4x² + 8xy + 6y² = 30 by applying a change of variables defined as x = u cos(θ) - v sin(θ) and y = u sin(θ) + v cos(θ). The goal is to determine the angle θ that will make the coefficient of the uv term equal to zero. Participants suggest substituting the new variables into the equation and solving for θ to achieve this elimination.

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Homework Statement



If you make the change of variables:

x= ucos(\theta)-vsin(\theta)
y= usin(\theta)+vcos(\theta)

where the angle 0 \leq \theta <\pi/2 is chosen in order to eliminate the cross product term in:

4x2+8xy+6y2=30

What is the angle you would use?

Homework Equations





The Attempt at a Solution


I have no idea. Would you find when the 8xy term (in terms of the new variables) is zero (solve for theta)?
 
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Not just the xy term. Each of the three terms will have "uv" in them.

Replace x with u cos(\theta)- v sin(\theta) and y with u sin(\theta)+ v cos(\theta) in 4x^2+ 8xy+ 6 y^2. There will be "uv" terms in all three of those terms. Add then up and choose \theta to make the coefficient of uv equal to 0.
 

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