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
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
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