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Cos x + cos 9x= cos 5x + cos 7x.
Cos 6x= ?

Răspuns :

MFMask
cos x+cos y=2cos(x+y)/2cos(x-y)/2
cos9x+cosx=2cos5xcos4x
cos5x+cos7x=2cos6xcosx
2cos5x cos4x=2cos6x cosx
cos 6x=cos5x.cos4x/cosx
dar cosxcosy=[cos(x+y) +cos(x-y)]/2
cos5xcos4x=(cos9x+cosx)/2
inlocuind
cos6x=cos9x/2cosx  +1/2