（cos85°+sin25°cos30°）/cos25°
= {cos(60°+ 25°) + sin25°cos30°} /cos25°
= {cos60°cos25° - sin60°sin25° + sin25°cos30°} /cos25°
= {cos60°cos25° - sin25°(sin60° - cos30°) } /cos25°
= cos60°cos25° / cos25°
= cos60°
= 1/2