解:(1)12:36:84,
=(12÷12):(36÷12):(84÷12),
=1:3:7;
(2)1
![](http://thumb.1010pic.com/pic5/latex/36.png)
:
![](http://thumb.1010pic.com/pic5/latex/2405.png)
:0.75,
=
![](http://thumb.1010pic.com/pic5/latex/15.png)
:
![](http://thumb.1010pic.com/pic5/latex/2405.png)
:
![](http://thumb.1010pic.com/pic5/latex/365.png)
,
=(
![](http://thumb.1010pic.com/pic5/latex/15.png)
×20):(
![](http://thumb.1010pic.com/pic5/latex/2405.png)
×20):(
![](http://thumb.1010pic.com/pic5/latex/365.png)
×20),
=24:6:15,
=8:2:5;
(3)4
![](http://thumb.1010pic.com/pic5/latex/13.png)
:3
![](http://thumb.1010pic.com/pic5/latex/8.png)
:5
![](http://thumb.1010pic.com/pic5/latex/96.png)
,
=
![](http://thumb.1010pic.com/pic5/latex/190.png)
:
![](http://thumb.1010pic.com/pic5/latex/395.png)
:
![](http://thumb.1010pic.com/pic5/latex/31142.png)
,
=(
![](http://thumb.1010pic.com/pic5/latex/190.png)
×12):(
![](http://thumb.1010pic.com/pic5/latex/395.png)
×12):(
![](http://thumb.1010pic.com/pic5/latex/31142.png)
×12),
=54:40:63.
分析:根据比的基本性质,即比的前项和后项同时乘或除以一个相同的数(0除外)比值不变,进而把比化成最简比.
点评:此题考查化简比的方法,注意化简比的结果仍是一个比,它的前项和后项都是整数,并且是互质数;还要注意:如果比的前后项是名数时,要把单位化统一后再化简比.