tan((α+β)/2)
=tan(α-β/2+β-α/2)
=[(tan(α-β/2)+tan(β-α/2)]/[1-(tan(α-β/2)*tan(β-α/2)]
=(1/2-1/3)/(1+1/2*1/3)
=1/6/(7/6)
=1/7