tana=1/2,tan(a-p)=-2/5

　　tan(2a-p)

　　=tan[a+(a-p)]

　　=[tana+tan(a-p)]/[1-tanatan(a-p)]

　　=(1/2-2/5)/(1+1/2*2/5)

　　=(5-4)/(10+2)

　　=1/12