cot2X=-2根号2
则:1/tan2x=-2根号2
1/[2tanx/(1-tan^2(x))]=-2根号2
[1-tan^2(x)]/(2tanx)=-2根号2
1-tan^2(x)=-4根号2tanx
tan^2(x)-4根号2tanx-1=0
则:tanx=2根号2+3 或 2根号2-3
又225度