tan（α＋β）*(1－tanα ·tanβ )＝tanα＋tanβ
首先我们看-(x/2-π/4)=π/2-(x/2+π/4)
tan(x/2+π/4)+tan(x/2-π/4)=tan((x/2+π/4)+(x/2-π/4))(1-tan(x/2+π/4)*tan(x/2-π/4)=tanx*(1-tan(x/2+π/4)*tan(-(x/2+π/4))=tanx*(1-(-1))=2tanx
所以得证