sin7xsin9x-sin2xsin4x=0
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1/2cos2x-1/2cos16x-1/2cos2x+1/2cos6x=01/2cos6x-1/2cos16x=01/2(cos6x-cos16)=01/2*2sin5xsin11x=0sin5x=0⇒5x=πn⇒x=πn/5,n∈zsin11x=0⇒11x=πn⇒x=πn/11,n∈z