f(x)=sin(3x+π/4),求f(π/9)的值第二小问：f（a/3+π/4）=1/2,a∈（π,3π/2）,求cos2a

f(x)=sin(3x+π/4),求f(π/9)的值第二小问：f（a/3+π/4）=1/2,a∈（π,3π/2）,求cos2a
f(x)=sin(3x+π/4),求f(π/9)的值
第二小问：f（a/3+π/4）=1/2,a∈（π,3π/2）,求cos2a

f(x)=sin(3x+π/4),求f(π/9)的值第二小问：f（a/3+π/4）=1/2,a∈（π,3π/2）,求cos2a
f(π/9)=sin(3π+π/4）=sin(π+π/4）=-sinπ/4=-√2/2
f（a/3+π/4）=1/2=cos(π/3）
a/3+π/4=π/3
a/3=π/12
a=π/4
则cos2a=0

cos2a=1/2
采纳，可以追问
f（a/3+π/4）=sin(3（a/3+π/4）+π/4)=-sina=1/2
sina=-1/2
a∈（π，3π/2)
a=π7/6
2a=π14/6=π7/3
cos2a=cosπ7/3=cosπ/3=1/2