(sin2 α)^2=(17/39)^2
(cos2α)^2=1-(sin2 α)^2=1-(17/39)^2
cos2α=(√(39^2-17^2))/39
39cos2α=39*(√(39^2-17^2))/39=√(39^2-17^2)=√(1521-289)=√1232=4√77
