Вычислите cos⁡50°-cos⁡40°∙ctg⁡115°

cos50 - cos40*ctg115 = cos50 - cos(90 - 50)*ctg(90 + 25) = = cos50 + sin50*tg25 = = cos50 +(sin50*sin25)/ cos25 =  = (cos50cos25 + sin50*sin25)/ cos25 = = cos(50-25)/cos25 =  = cos25/cos25 = 1