1)cos²a-tga*ctga=cos²a-1=-sin²a
2)(1-sin²a)/sin²a=cos²a/sin²a=ctg²a
3)(1/sin²a -1)*tg²a=(1+ctg²a-1)*tg²a=ctg²a*tg²a=1
4)tga*ctga-(ctga*sina)²=1-cos²a=sin²a
5)tgactga+ctg²a=1+ctg²a=1/sin²a
6)ctga/tga +sin²a+cos²a=ctg²a+1=1/sin²a
7)1/cos²a -tg²a-sin²b=1+tg²a-tg²a-sin²b=cos²b
8)(tg²a-sin²a)(1/sin²a -1)=(tg²a-sin²a)*ctg²a=1-cos²a=sin²a
9)sina/(1-cosa) -(1+cosa)/sina=(sin²a-1+cos²a)/sina(1-cosa)=0
10)(sin²a + 1/(1+tg²a)*ctga=(sin²a+cos²a)*ctga=ctga
11)cos^6a+sin^6a -(3sin²a-3)/(1+ctg²a)=cos^6a+sin^6a +3cos²a*sin²a=
(1+cos2a)³/8 +(1-cos2a)³/8+3(1-cos2a)(1+cos2a)/4=
=1/8(1+3cos2a+3cos²2a+cos³2a+1-3cos2a+3cos²2a-cos³2a+6-6cos²2a)=
1/8*8=1