If (1−sinA)(1−sinB)(1−sinC)=(1+sinA) (1+sinB)(1+sinC) , then each sides is equal to
±cosAcosBcosC
0
±sinAsinBsinC
1
Multiply both the sides by
(1−sinA)(1−sinB)(1−sinC) to obtain
(1−sin2A) (1−sin2B) (1−sin2C) =cos2Acos2Bcos2C⇒(1−sinA)(1−sinB)(1−sinC)=±cosAcosBcosC