x²-4x+4+y²-9=0
(x-2)²+y²=9
设x=3cost+2,y=3sint
Y-X=3sint-3cost-2=3(sint-cost)-2
=3√2(√2/2sint-√2/2cost)-2
=3√2sin(t-π/4)-2
-1采用的是什么思想