[email protected](6п - @)(1 + ctg^2([email protected])=ctg([email protected])

Cosα*sin(6π-α)*(1+ctg²(-α))=ctg(α)
1. sin(6π-α)=-sinα
2. 1+ctg²(-α)=1+ctg²α=1/sin²α
3. cosα*(-sinα)/sin²α=-cosα/sinα=-ctgα
4. ctg(-α)=-ctgα
-ctgα=-ctgα