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
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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