原式=sin(a-7π/3)+cos(a+13π/6)=sin(a-π/戚仔早3-2π)+cos(a+π/6+2π)=sin(a-π/3)+cos(a+π/戚晌6)=sin(a-π/3)+sin(π/高雀2-a-π/6)=sin(a-π/3)+sin(π/3-a)0