已知f（x）=4sin（3x+π/4）,若f（2/3a+π/12）=12/5,求sina的值

已知f（x）=4sin（3x+π/4）,若f（2/3a+π/12）=12/5,求sina的值
已知f（x）=4sin（3x+π/4）,若f（2/3a+π/12）=12/5,求sina的值

已知f（x）=4sin（3x+π/4）,若f（2/3a+π/12）=12/5,求sina的值
【解】f(2/3a＋π/12)=12/5
==>4sin[3(2/3a＋π/12)＋π/4)]=12/5
==>4sin(a＋π/4＋π/4)=12/5
==>4sin(a＋π/2)=12/5
==>－4sina=12/5
==>sina=－3/5

就打入进去就行了。4sin（2a+π/2）=12/5
-4cos 2a=12/5