Решение
1) cosα = 12/13
sinα = - √(1 - cos²α) = - √ (1 - (12/13)²) = - √25/169 = - 5/13
tgα = sinα / cosα = - (5/13) : (12/13) = - 5/12
ctgα = 1/tgα = - 12/5
2) 3 - sin²x - cos²x = 3 - (sin²x + cos²) = 3 - 1 = 2
3) (cos²α - 1) / (sinα - 1) + tg(1/8)*ctg(1/8) = - (1 - cos²α)/(sinα - 1) + 1 =
= - sin²α / (sinα - 1) + 1 = - (sin²α + sinα - 1) / (sinα - 1)