sina<0
sin²a+cos²a=1
sina=-3/5
原式=(1+sin(a/2)/cos(a/2))/(1-sin(a/2)/cos(a/2))
上下乘cos(a/2)
=[cos(a/2)+sin(a/2)]/[cos(a/2)-sin(a/2)]
上下乘[cos(a/2)+sin(a/2)]
=[cos(a/2)+sin(a/2)]²/[cos(a/2)-sin(a/2)][cos(a/2)+sin(a/2)]
=(1+sina)/(cosa)
=-1/2