cos80°cos55°-cos10°cos35°
=cos80°cos55°-cos(90°-80°)cos(90°-55°)
=cos80°cos55°-sin80°sin55°
=cos(80°+55°)
=cos(90°+45°)
= -cos45°
= -√2/2.=cos80°cos55°-cos(90°-80°)cos(90°-55°)这步是如何到=cos80°cos55°-sin80°sin55°利用cos(90°-θ)=sinθ即得。