1) соs 2a = 1 - 2sin^2 a = 1- 2(-8/17)^2 = 161/289
2) Ctg (π|2 +α) = - tg a
tg a = sin a / cos a
cos a = - корень (1-(-8/17)^2)=-15/17
tg a = (-8/17)/( -15/17 )=8/15
Ctg (π|2 +α) = -8/15
