sina+sinb+siny=0
-sina=sinb+siny
两边平方:
(sina)^2=(sinb)^2+2sinbsiny+(siny)^2--------1
cosa+cosb+cosy=0
-cosa=cosb+cosy
两边平方:
(cosa)^2=(cosb)^2+2cosbcosy+(cosy)^2--------2
1+2:
2(sinbsiny+cosbcosy)=-1
2cos(b-y)=-1
cos(b-y)=-1/2
sina+sinb+siny=0
-sina=sinb+siny
两边平方:
(sina)^2=(sinb)^2+2sinbsiny+(siny)^2--------1
cosa+cosb+cosy=0
-cosa=cosb+cosy
两边平方:
(cosa)^2=(cosb)^2+2cosbcosy+(cosy)^2--------2
1+2:
2(sinbsiny+cosbcosy)=-1
2cos(b-y)=-1
cos(b-y)=-1/2