If the form of a conic is
ax˛ + 2hxy + by˛ + 2fx + 2gy + k = 0
and to rotate the axis from (x,y) to (X,Y) you must use
x = XcosT - YsinT
y = XsinT + YcosT
how can I show that to get rid of the xy coefficient, T must be found using
tan(2T) = 2h / (a-b)???