关于三角函数的小题~
cos40°+cos60°+cos80°+cos160°可化简为 A.0 B.1/2 C.1/2+2cos20° D.-1
B cos40°+cos60°+cos80°+cos160 =cos(60°-20°)+cos60°+cos(60°+20°)+cos(180°-20°) =2cos60°*cos20°+cos60°+cos(180°-20°) =cos20°+1/2-cos20° =1/2
cos40+cos60+cos80+cos160 = cos(60-20)+cos60+cos(60+20)+cos(180-20) = cos60cos20+sin60sin20+cos60+cos60cos20-sin60sin20+cos180cos20+ sin180sin20 =1/2 cos20 + 1/2 + 1/2 cos20 -cos20 +0 =1/2 参考公式: cos(A+B) = cosAcosB - sinAsinB cos(A-B) = cosAcosB + sinAsinB
答:2*cos20*cos40*cos80= 2*sin20cos20*cos40*cos80/sin20= sin40cos40*cos80/sin20= 2si...详情>>
答:是"我们"的宾格,所以是"us". Let us go! Do you know?详情>>