a/b=c/d
[a/b+1]/√[(a/b)^2+1] =[c/d+1]/√[(c/d)^2+1] ==》
[a/b+1]/[c/d+1] =√[(a/b)^2+1] /√[(c/d)^2+1] ==》
a,b均为正数 ,
[a+b]/[c+d] =√[a^2+b^2] /√[c^2+d^2]
a,b均为负数
b[a/b+1]/d[c/d+1] =b√[(a/b)^2+1] /d√[(c/d)^2+1]
[a+b]/[c+d] =（-√[a^2+b^2] ）/（-√[c^2+d^2]）
=[a+b]/[c+d] =√[a^2+b^2] /√[c^2+d^2]