cos的除數問題

1-cosX=-sinX

1-2cosX-2cos(2)X=sin(2)X

1-2cosX+2cos(2)X-[1-cos(2)X]=0

2cosX+2cos(2)X=0

點解下一步

2cosX=-2cos(2)X

1=-cosX

係錯?

注:(2)<-2次方

Comments

  • 錯的原因是若X=90/270....., cosX = 0,那樣便不能兩邊一起除cosX,因除0是undefined的.

  • From 2cosX+2cos(2)X=0,

    2cosX (1+cosX)=0

    ie: 2cosX=0 or 1+cosX=0

    Ans: X=90 degree or (pi)/2 or X= 180 degree or pi

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