cosb.coscsinb.sinc+cosc.cosasinc.sina+cosa.cosbsina.sinb= Payda eşitlenirse,
=sina.cosb.cosc+sinb.cosc.cosa+sinc.cosa.cosbsina.sinb.sinc
=cosc(sina.cosb+sinb.cosa)+sinc.cosa.cosbsina.sinb.sinc
=cosc.sin(a+b)+sinc.cosa.cosbsina.sinb.sinc a+b+c=π⇒a+b=π−c dir.
=cosc.sin(π−c)+sinc.cosa.cosbsina.sinb.sinc sin(π−c)=sinc dir.
=cosc.sinc+sinc.cosa.cosbsina.sinb.sinc=sinc(cosc+cosa.cosb)sina.sinb.sinc
=cosc+cosa.cosbsina.sinb=cos(π−(a+b))+cosa.cosbsina.sinb cos(π−(a+b))=−cos(a+b)=−(cosacosb−sina.sinb) dir.
=−cos(a+b)+cosa.cosbsina.sinb=−cosa.cosb+sina.sinb+cosa.cosbsina.sinb=sina.sinbsina.sinb=1 olacaktır.