sin8A sin4A - sin7A sin3A = sin11A sinA
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LHS =sin8A.sin4A -sin7A.sin3A
=> 1/2{cos(8A-4A) -cos(8A+4A)} - 1/2{cos(7A-3A)-cos(7A+3A)}
=>1/2{cos4A-cos12A-cos4A+cos10A}
=>1/2{cos10A - cos12A }
=>1/2{2sin(10+12)A/2.sin(12-10)A/2}
=>sin11A.sinA = RHS
=> 1/2{cos(8A-4A) -cos(8A+4A)} - 1/2{cos(7A-3A)-cos(7A+3A)}
=>1/2{cos4A-cos12A-cos4A+cos10A}
=>1/2{cos10A - cos12A }
=>1/2{2sin(10+12)A/2.sin(12-10)A/2}
=>sin11A.sinA = RHS
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