Monday, 20 May 2013

Substitution: so what does it all mean?




Substitution: so what does it mean?

 

Substitution is taught every year and yet it is always causing problems. In particular when we ask a student to substitute a number into 2a then a2.It is not the act of substituting that causes the problem but the understanding of algebra. Common misconceptions when substituting into a term happens time and time again. This activity helps to identify those that do, and do not understand, algebra particularly when those difficult areas of powers and brackets are used.

 

Step 1

A bit of preparation, but once done it is a resource that can be used time and time again. Each pupil needs a set of cards numbered 0 to 9. Make them big get them laminated, held together with an elastic band and stored in a cheap envelope.

 

Step2

Hand out a set of cards to each pupil or give them a set between two if you want a more collaborative approach. Write an expression on the board such as

 

2a

 

Emphasise that this is an expression not an equation. You may have to tell them what the difference is, again another big problem what is an expression, what is an equation.

 

Step 3

Now tell them a = 4, using the cards show the answer. Hopefully they will all demonstrate they know the answer is 8, if not you have an opportunity to rectify any misconceptions. Now put the following on the board

 

a2

 

Once again the same process is followed you can ask the class show me the answer. It is now some confusion occurs, some will give you the answer 16, others 8 again. Now is the opportunity to find out who understands and who doesn’t. Dependent on the answers it is up to you how you progress. Try other numbers and other powers. This quick simple exercise will inform your assessment and guide future lessons.

 

Extension

Once you have successfully rectified any problem with these issues, and it is a major problem to be solved if you want them to progress in algebra, you can now try something like these more challenging terms 3a2 and (3a)2. This starting activity will provide the basis for much useful discussion.