Question

1.化简sin25/12πsin11/6π－cos25/12πcos11/6π的值是？

Answer 1

sin(a+b)=sin(a)*cos(b)+cos(a)*sin(b)
sin(a-b)=sin(a)*cos(b)-cos(a)*sin(b)
cos(a+b)=cos(a)*cos(b)-sin(a)*sin(b)
cos(a-b)=cos(a)*cos(b)+sin(a)*sin(b)
那么
1,sin25/12πsin11/6π-cos25/12πcos11/6π
= - cos(25/12π+11/6π)
= - cos(47/12π)
2,sina=15/17 得到 cosa=√(1-(15/17)^2)=8/17
cos(a-π/3)
= cos(a)cos(π/3) + sind(a)sin(π/3)
=8/17 * 1/2 + 15/17 * √3/2
=4/17+15√3/34,1,
huiminhou 举报
2.cosa是第二象限的角呢 哦。忘记了，那就前面加个负号，cosa=-8/17, 再代入式中。 -4/17+15√3/34,1.化简sin25/12πsin11/6π－cos25/12πcos11/6π的值是
2.已知sina=15/17,a是第二象限角,则cos[a－π/3]= cos[a+π/3]=