Докажите тождество
[tex](ctg^2a-tg^2a)* \frac{1}{ctg^a-1} = \frac{1}{cos^2a} [/tex]

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