三角函数题,帮我做做~若∏≤a≤2∏,且sinasin3a≤0,则a满足.终于找到答案了，是240°＜a＜300°~~

Question

三角函数题,帮我做做~
若∏≤a≤2∏,且sinasin3a≤0,则a满足_________.
终于找到答案了，是 240°＜ a ＜ 300°~~

Answer 1

这题是可以解出具体值的,一楼来猜的不可以
不能这样糊弄楼主啊
sinasin3a=0.5[cos(a-3a)-cos(a+3a)]
=0.5[-cos2a+cos4a]
=0.5[-cos2a+2cos^2(2a)-1]
=[cos^2(2a)-0.5cos2a-1/2]
=[(cos2a-1/4)^2-1/4-1/2]
=(cos2a-1/4)^2-3/4