Andrew R. answered 07/06/18
Tutor
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PhD in Mathematical Physics
5x2-6xy+5y2=12 Note that this eq. is symmetric in x and y which means it's symmetric to the line y=x, so look at it in an x'y'-coordinate system rotated by 45°.
(Let θ=45° in the transformation matrix R, for X=RX')
Substitute x=(1/√2)(x'-y'), y=(1/√2)(x'+y') and get
x'2+4y'2=6