3) cos2α / (cosα-sinα) = (cos²α-sin²α) / (cosα-sinα) = (cosα-sinα)(cosα+sinα) / (cosα-sinα) = cosα+sinα.
4) (2cos10° * cos70° - cos80°) / (2sin40° * cos10° - sin50°) = (cos80° + cos60° - cos80°) / (sin50° + sin30° - sin50°) = cos60° / sin30° = 1/2 : 1/2 = 1.
