cos2θ=-1/2
â´2θ=kÏ + 2Ï/3ï¼kâZ
â´Î¸=kÏ/2 + Ï/3ï¼kâZ
âµ0â¤Î¸â¤180º
â´å½k=0æ¶ï¼Î¸=Ï/3
å½k=1æ¶ï¼Î¸=5Ï/6
å2θ=5Ï/3ï¼cos(5Ï/3)=1/2ï¼ä¸å·²ç¥ä¸ç¬¦ï¼èå»
å½k=-1æ¶ï¼Î¸=-Ï/6ï¼ä¸å·²ç¥ä¸ç¬¦ï¼èå»
â´Î¸=Ï/3
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