tan15°+cos15°/sin15°=sin15°/cos15°+cos15°/sin15°
(sin15°)^2+(cos15°)^2 1 1 1
=-------------------------------=-----------------------=-----------------=------------------=4
sin15°cos15° sin(2*15° )/2 sin(30° )/2 (1/2)/2
(sin2α=2sinαcosα)