令a/b=c/d=k,解出a=bk,c=dk,将其带入则a+c=(b+d)k.所以(a+c)/(b+d)=k,则得证。。后边那个也一样的。。
等比性质
如果a/b=c/d=…=m/n(b+d+…+n≠0),那么
(a+c+…+m)/(b+d+…+n)=a/b=c/d…=m/n
证明:设a/b=c/d=…=m/n = k
则a = bk, c = dk,…m = nk
则(a+c+…+m)/(b+d+…+n) = (bk + dk +...+ nk)/(b+d+…+n) = k = a/b
比例性质
设a/b=c/d=f/e=k
代入,即可