三角函数.

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三角函数.

三角函数.
三角函数.
 

三角函数.
18.(A).
19.sin(13π/6)=sin(2π+π/6)=sinπ/6=(1/2);
cos9π/4=cos(2π+π/4)=cosπ/4=(√2/2);
sin(-2π/3)=-sin2π/3=(-√3/2);
20.sin(-11/6)=-sin(11π/6)=-sin(2π-π/6)=sinπ/6=(1/2);
cos(-7π/3)=cos(7π/3)=cos(2π+π/3)=cosπ/3=(1/2);
cos(-11π/3)=cos11π/3)=cos(4π-π/3)=cosπ/3=(1/2);
21.cos750°=cos(2*360°+30°)=cos30°=(√3/2);
cos1500°=cos(4*360°+60°)=cos60°=(1/2);
22.sin(-3π/4)=-sin(3π/4)=-√2/2;
cos(-4π/3)=cos(4π/3)=cos(π+π/3+-cosπ/3=(-1/2);
sin(-5π/3)=-sin5π/3=-sin(π+2π/3)=sin2π/3=(√3/2).