Question

Тригонометрия! Помогите! 

Answer 1

2$frac{2sin^2 70 -1}{2ctg 115 * cos^2 155} = frac{-cos (2*70)}{2tg(90+25)*cos^2(180-25)}= frac{-cos140}{2 frac{sin25}{cos25} * cos^2 25 } = frac{-cos140}{2sin25*cos25}=$ $frac{-cos140}{sin50}= frac{cos(180-40)}{sin(90-40)}= frac{-cos40}{cos40}=-1$