Step 7 Objective

Use logical argument to establish the truth of a statement; begin to give mathematical justifications and test by checking particular cases.

Examples of what pupils should know and be able to do

Hollow Squares

Here is a hollow square.

diagram showing the hollow square created by circling dots on squared paper.
  • How many pegs form the square on the outside?
  • How many pegs are there in the hollow?
  • Draw some more hollow squares.
  • Investigate.
diagram showing how the formula 2x+2(x-2) equals number of pegs around the outside of a square and to work out the hollow would be (x-1)squared equals the size of the hollow, where x is equal to the number of pegs along the top side.

Examples drawn from Hollow Squares.

As in Step 6 but with mathematical explanations about the patterns in the results.


Probing questions

How would you convince a friend... a penpal?

How would you convince me?

Does this work for any value? How do you know?

What special cases should you check?

What if pupils find this a barrier?

diagram showing three lines crossing

Line Crossings

  • Draw three straight lines (line segments) so that some cross over each other.
  • How many crossings are there?
  • Try different arrangements of the lines. What is the maximum number of possible crossings?
  • Try using more lines.
  • Is there a rule for the maximum for any number of lines? If so, write it down.

Use the problem Line Crossings.

  • Put all of your results (for greatest number of crossings) into a table.
  • What patterns can you see? How would you explain them?
  • Look at your table/pattern, how might you know if some of your results are wrong?

With 5 lines I can draw a lot of patterns with different number of crossings.

When lines are parallel there are no crossings.

You have to be careful because if you make some lines longer they will cross other lines.

The more jumbled the picture the more crossings there are.


Multi  lines crossing