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cos九分之十三兀等于多少

cos13/9兀=cos(兀+4/9兀)=-cos2*2兀/3=-(cos2兀/3-sin2兀/3)=-(1/4-3/4)=1/2

cos13π=cos(12π+π)=cosπ=-1 cos13π/9=-1/9

1π90度 13π等於1170度 cos-13π等於cos-1170又等於cos-90等於0

cos(9π/2)=cos(2*2π+π/2),根据诱导公式,2π的整数倍都能约去 所以cos(9π/2)=cos(π/2)=0

解:令cos(π/13)+cos(3π/13)+cos(9π/13)=x,则x^2=[cos(π/13)]^2+[cos(3π/13)]^2+[cos(9π/13)]^2+2cos(π/13)cos(3π/13)+2cos(3π/13)cos(9π/13)+2cos(9π/13)cos(π/13)=1/2[1+cos(2π/13)]+1/2[1+cos(6π/13)]+1/2[1+cos(18π/13)+cos(2π/13)+cos(4π/13)+

解:cos(-9分之11π)=cos(9分之11π)≈-0.766044

cos(2013π)=cos(π)= -1

cos(13π/4)=cos[3π+(π/4)]=-cos(π/4)=-√2/2

cos13π/9=cos(2π-5π/9)=cos5π/9 =cos(π/2+π/18) =-sinπ/18.

(根2)/2 不用谢,望采纳

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