Explanation:
If cos x = 5/13 and x is between 0 and 90 degrees, since we know that tan2x = sin2x/cos2x = (2 sinxcosx)/(cos^2x - sin^2x) and we are dealing with a 5-12-13 right triangle,
tan (2x) = 2(12/13)(5/13)/((5/13)^2 - (12/13)^2) = (2*12*5/13^2) / (25/13^2) - (144/13^2)= 120/(-119)= -1.008403