Question
If cos 39° = x/ √(x2 + y2),Â
 then the value of tan 51° = ?Solution
cos 39° = x/ √(x 2 + y 2) =  Base/hypotenuse So base = x & hypotenuse =Â
so perpendicular = y But for angle 51° which is complimentary of 39° , perpendicular =y & base = x. So tan 51° = base/perpendicular =Â
Alternate Method : cos 39° =Â
OR sin (90°-39°) =Â
OR sin 51° =Â
  -----(1) Squaring on both sides, we will get, sin2 51° =Â
or 1 - cos2 51° =Â
or cos2 51° = 1 -Â
=Â
So cos 51° = Â
----(2) Hence tan 51° = sin51°/ cos51° =

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