255.ctgα-tgv/ctgv+tgα=cos2α
cosα/sinα-sinα/cosα cosαsinα+sinα/cosα=cos²α-sin²α/ /sinαcosα/cos²α+sin²α/sinαcosα=cosvsin²α/1=cos2α 277.(1+ctg²α)(1-sin²α)=ctg²α (1+cos²α/sin²α)cos²α=ctg²α sin²α+cosv/sinv*cos²α=ctg²α cos²α=ctg²α ctg²α=ctg²α 278.1/1+tang²d+1/1+crg²d=1 1/1+sin²α/cos²α+1/1+cos²α/sin²α=1/cos²α+sin²α/cos²α+1/sin²α+cos²α/sin²α=cos²α/1+sin²α1=1