The answer i think is maybe 91/20 and 1/28
AAS, the two angles in the middle are vertical angles and are therefore congruent :)
tan x = sin x/cos x
cos³ x *tan²x - cos x = cos³x *(sin²x/ cos² x) - cos= cosx*sin²x - cos x =
= cos x(sin² x - 1) = - cos x(1 - sin²x)= - cosx*cos²x = - cos³ x
