17cos(2arctg(-1/4)) ; 75cos(2 arccos 4/5)

17cos(2arctg(-1/4))=17cos2t=17*(cos^2 t -sin^2 t)=17*(16/17-1/17)=15;
t=arctg(-1/4)
tgt=-1/4;  1+tg^2 t=1/cos^2 t
                1+(-1/4)^2=1/cos^2 t;  1+1/16=(16+1)/16=17/16
                1/cos^2 t=17/16;  cos^2 t=16/17
                                           sin^2 t+cos^2 t=1;  sin^2 t=1-cos^2 t; sin^2 t=
=1-16/17=1/17