关于y轴对称,即为偶函数.
f(-x)=f(x)
即3sin(-2x+5θ)=3sin(2x+5θ)
sin(5θ-2x)=sin(5θ+2x)
展开得：
sin5θcos2x- cos5θsin(2x)= sin5θcos2x+cos5θsin(2x)
2cos5θsin(2x)=0,
即cos5θ=0,
5θ=kπ+π/2,k∈Z.
θ= kπ/5+π/10,k∈Z.