1/(2*sin270°)-(2cos40°)/1=(1-4cos20°*cos40°)/(2cos20°)=
(1-4*1/2(cos60°+cos20°))/(2cos20°)=(1-2*(1/2+cos20°))/(2cos20°)=
((1-1-2cos20°))/2cos20°=-cos20°/cos20°=-1
Дoказано. Использовал формулы замены sin70°=cos20°
cosα*cosβ=1/2*((cos(α+β)+cos(α-β))
1/(2*sin270°)-(2cos40°)/1=(1-4cos20°*cos40°)/(2cos20°)=
(1-4*1/2(cos60°+cos20°))/(2cos20°)=(1-2*(1/2+cos20°))/(2cos20°)=
((1-1-2cos20°))/2cos20°=-cos20°/cos20°=-1
Дoказано. Использовал формулы замены sin70°=cos20°
cosα*cosβ=1/2*((cos(α+β)+cos(α-β))