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alexeymak
@alexeymak
August 2022
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Cos2xcos7x-sin2xsin7x=-1
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Nekha
Cos2xcos7x-sin2xsin7x=-1
Cos(a+b)=-1
cos(2+7)x=-1
cos9x=-1
9x=arccos(-1)+п
9x=2пк+п
х=2пк/9+п/9
x=1/9(2пк+п)
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ulasovage
Cos2xcos7x=1/2(cos(2x-7x)+cos(2x+7x) )=1/2(cos(-5x)+cos9x)=1/2(cos5x+cos9x)
sin2xsin7x=1/2(cos(2x-7x)-cos(2x+7x) )=1/2(cos(-5x)+cos9x)=1/2(cos5x+cos9x)
cos2xcos7x-sin2xsin7x=1/2(cos5x+cos9x-cos5x-cos9x)=1/2*0=0
cosx- функция чётная.поэтому cos(-5x)=cos5x
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Answers & Comments
Cos(a+b)=-1
cos(2+7)x=-1
cos9x=-1
9x=arccos(-1)+п
9x=2пк+п
х=2пк/9+п/9
x=1/9(2пк+п)
sin2xsin7x=1/2(cos(2x-7x)-cos(2x+7x) )=1/2(cos(-5x)+cos9x)=1/2(cos5x+cos9x)
cos2xcos7x-sin2xsin7x=1/2(cos5x+cos9x-cos5x-cos9x)=1/2*0=0
cosx- функция чётная.поэтому cos(-5x)=cos5x