Докажите тождество
Ctg²a-1/ctg²a=(ctg^4a-1)/ctg²a (ctg^4a-1)/ctg²a *1/(ctg²a-1)=(ctg²a-1)(ctg²a+1)/ctg²a*1/(ctg²a-1)= =(ctg²a+1)/ctg²a=1/sin²a:cos²a/sin²a=1/cos²a