Step-by-step explanation:
sin(s+t) = sin(s)+cos(t) which results
(sin(s)×cos(t))/(cos(s)cos(t)) + (cos(s)×sin(t))/(cos(s)cos(t))
knowing that tan=sin/cos and that cos/cos=1 and sin/sin=1
resulting tan s ×1 + tan t × 1 = tan s + tan t
