COROANA!!!
ne folosim de regula fundamentala UTILA IN ORICE CALCUL TRGINOMETRIC
sinus la patrat de x + cosinus la patrat de x = 1
vom avea
sin^2 (x) + cos^2 (x) = 1
(12/13)^2 + cos^2 (x) = 1
144/169 + cos^2 (x) = 1
cos^2 (x) = 1 - (144/169) = (169-144)/169 = 25/169 => cos = 5/13
