3) sin^2a/cos^2a * cos^2a + cos^2a/sin^2a * sin^2a = sin^2a+cos^2a=1
2)ctga=8/15
tga*ctga=1
tga=1*15/8
1+tg^2a= 1/ cos^2a
1+225/64 = 1/cos^2a
289/64=1/cos^2a
cos^2a=64/289
cosa=8/17
sina=√1-64/289 = √225/289=15/17
1)sin^2a/cos^2a - sin^2a = sin^2a * sin^2a/cos^2a
sin^2a-sin^2acos^2a / cos^2a = sin^4a/cos^2a
sin^2a(1-cos^2a)/cos^2a= sin^4a/cos^2a
sin^2a*sin^2a/cos^2a = sin^4a/cos^2a
sin^4a/cos^2a= sin^4a/cos^2a