根号[(x1)^2+1]-根号[(x2)^2+1] ={根号[(x1)^2+1]-根号[(x2)^2+1]}×{根号[(x1)^2+1]+根号[(x2)^2+1]}/{根号[(x1)^2+1]+根号[(x2)^2+1]} =[(x1)^2+1-[(x2)^2-1]}/{根号[(x1)^2+1]+根号[(x2)^2+1]} =(x1+x2)(x1-x2)/{根号[(x1)^2+1]+根号[(x2)^2+1]}
分子分母同除以cos²α, 可得