history of mathematics - National STEM Centre
history of mathematics - National STEM Centre
history of mathematics - National STEM Centre
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12 Summaries and exercises<br />
• extending the number line to include complex numbers (Activity 11.3)<br />
• looking at an axiomatic approach (Activities 11.5 to 11.7)<br />
• Boolean algebra as an example <strong>of</strong> an axiomatic system (Activity 11.8)<br />
• exploring Clifford algebra (Activities 11.9 to 11.14)<br />
• generalising beyond numbers (Activity 11.15).<br />
Practice exercises<br />
1 In his Arithmetica infinitorum, John Wallis gives his view that a number larger<br />
than infinity can be found.<br />
I<br />
a ratio greater than infinity such as a positive number may be supposed to<br />
have to a negative number<br />
a Investigate the sequence <strong>of</strong> positive ratios — — — —. This is a sequence <strong>of</strong> the<br />
form — with n increasing. In numerical terms is the sequence descending or<br />
n<br />
ascending?<br />
b Now investigate the sequence <strong>of</strong> ratios — where n is any positive number. What<br />
n<br />
can you say about the sequence as n tends towards zero? Draw a graph using n as<br />
the horizontal axis.<br />
c By extending the sequence to negative values <strong>of</strong> n, again with n decreasing, can<br />
you explain why John Wallis may have put forward the view expressed?<br />
d What is wrong with Wallis's argument?<br />
2 In the book Mathematical thought from ancient to modern times, Kline assigns<br />
to Augustus de Morgan the following:<br />
The imaginary expression -^(-a) and the negative expression -b have<br />
this resemblance, that either <strong>of</strong> them occurring as the solution <strong>of</strong> a<br />
problem indicates some inconsistency or absurdity. De Morgan illustrated<br />
this by means <strong>of</strong> a problem. A father is 56; his son is 29. When will the<br />
father be twice as old as the son?<br />
a Solve de Morgan's problem by setting up an equation to find out how many<br />
years from now the father will be twice as old as the son.<br />
b In what way does your solution support de Morgan's claim?<br />
c How do you interpret your solution and what assumptions are you making about<br />
numbers?<br />
d How can de Morgan's original question be phrased so that the solution no longer<br />
appears absurd?<br />
3 Discuss critically various contradictory beliefs which have been held about<br />
negative numbers throughout the centuries and throughout the world.<br />
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