设a=cos50°cos127°+cos40°cos37°b=根号2\2（sin56°-cos56°）c=1-tan^239°\1+tan^239°d=（cos80°-2cos50°+1）\21楼我你本来应该答四个但是你只答了1个还错了。

Question

设a=cos50°cos127°+cos40°cos37°
b=根号2\2（sin56°-cos56°）
c=1-tan^2 39°\1+tan^2 39°
d=（cos80°-2cos50°+1）\2
1楼我你本来应该答四个但是你只答了1个还错了。

Answer 1

a=cos50°cos127°+cos40°cos37°
=cos50°cos(90°+37°)+cos(90°-50°)cos37°
= cos50°*[-sin37°]+sin50°cos37°
=sin50°cos37°-cos50°sin37°
=sin(50°-37°)
=sin13°
b=√2/2（sin56°-cos56°）
=√2/2*sin56°-√2/2*cos56°
=sin56°*cos45°-cos56°sin45°
=sin(56°-45°)
=sin11°
c=(1-tan^2 39°)/(1+tan^2 39°)
=cos(2*39°)
=cos(78°)
=cos(90°-12°)
=sin12°
第四题有误
d=（cos80°-2(cos50°)^2+1）/2
=[2（cos40°)^2-1-2(cos50°)^2+1]/2
=[2（cos40°)^2-2(cos50°)^2]/2
=（cos40°)^2-(cos50°)^2
=(cos40°)^2-[cos(90°-40°)]^2
=(cos40°)^2-(sin40°)^2
=cos(2*40°)
=cos80°
=cos(90°-10°)
=sin10°