1. (1-cos²α)/sinαcosα=sin²α/sinαcosα=sinα/cosα=tgα 2. sin²α(tgα+ctgα)=sin²α(sin²α+cos²α)/sinαcosα=sinα*1/cosα=tgα 3. tgα+tgα=2tgα 4. 2tgα=2tgα