The answer process is as follows:

According to the number of chickens and rabbits living in a cage, if there are X chickens, there are X rabbits.

According to the fact that chicken legs are less than rabbit legs 18, 4x-2x= 18, and x= 18/2=9.

Extended data:

One-dimensional linear equation can solve most engineering problems, travel problems, distribution problems, profit and loss problem, integral table problems, telephone billing problems and digital problems. If only arithmetic is used, some problems may be extremely complicated and difficult to understand.

The establishment of linear equation model will be able to find the equivalence relationship from practical problems and abstract it into a mathematical problem that can be solved by linear equations. For example, it may not be possible to start with algebraic expressions for the Diophantine problem, but finding "age" as an equivalent relationship through a linear equation will simplify the problem.

One-dimensional linear equations can also play a role in proving mathematical theorems, such as proving that "the period of 0.9 is equal to 1" within the scope of elementary mathematics. By verifying the rationality of the solution of linear equation, the purpose of explaining and solving life problems is achieved, and some problems in production and life are solved to some extent.

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