(y-sin2a)/(sin2b-sin2a)=(x-cos2a)/(cos2b-cos2a)
(y-sin2a)(cos2b-cos2a)=(sin2b-sin2a)(x-cos2a)
2(y-sin2a)sin(a+b)sin(a-b)=-2cos(a+b)sin(a-b)(x-cos2a)－－－和差化积公式（注意符号）
(y-sin2a)sin(a+b)=-cos(a+b)(x-cos2a）－－－－－条件是sin(a-b)≠0
ysin(a+b)-sin2asin(a+b)=-xcos(a+b)+cos(a+b)cos2a
xcos(a+b)+ysin(a+b)=cos(a+b)cos2a+sin2asin(a+b)＝cos(a+b-2a) --------余弦的和角公式
＝cos（b－a）＝cos(a-b)
所以xcos(a+b)+ysin(a+b)-cos(a-b)=0