(ab+bc+ca)/abc
=ab/abc+bc/abc+ca/abc 乘法分配律
=1/c+1/a+1/b 约分
=1/a+1/b+1/c
(a+b)/ab
=a/ab+b/ab 乘法分配律
=1/b+1/a 约分
=1/a+1/b