韦达定理得到：
tanx+tany=4p,
tanxtany=-3
tan(x+y)=(tanx+tany)/(1-tanxtany)=p,
cos^2(x+y)=1/(1+p^2)
tanx-tany=√(tanx+tany)^2-4tanxtany)=√16p^2+12
tan(x-y)=(tanx-tany)(1+tanxtany)=-√(p^2+3)
cos(x-y)=1/√(p^2+4)
2cos2xcos2y+2sin^2(x-y)
=2cos2xcos2y+1-cos(2x-2y)
=2cos2xcos2y+1-cos2xcos2y-sin2xsin2y
=cos2xcos2y-sin2xsin2y+1
=cos(2x+2y)+1
=2cos^2(x+y)
=2/(1+p^2)