If cos 39° = x/ √(x2 + y2), 

Solution

cos 39° = x/ √(x ² + y ²) =   Base/hypotenuse So base = x &  hypotenuse =  so perpendicular = y But for angle 51° which is complimentary of 39° , perpendicular =y & base = x. So tan 51° = base/perpendicular  =  Alternate Method : cos 39° =  OR  sin (90°-39°)  =  OR sin 51° =     -----(1) Squaring on both sides, we will get, sin² 51° =  or 1 - cos² 51° =  or cos² 51° = 1 -  =  So cos 51° =   ----(2) Hence tan 51° = sin51°/ cos51° =