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Chapter 4
Ancient and Modern Algebra
T HE EXACT SOLUTION OF P OLYNOMIAL E QUATIONS
In this chapter we turn our attention to exact solutions of polynomial equations.
The Babylonians used tables of values of n
3
+ n to find numerical solutions of easy
cubic equations, however the Greeks around the time of Euclid became obsessed with
exactness. Largely due to the dominant influence of Euclid’s Elements there was a
great interest in exact solutions up to the proof by Abel and Galois of the impossibility
of such solutions in general. In the first few sections of this chapter we review some of
the history of exact solutions. A good overview of this history is included in B.L van
der Waerden’s A History of Algebra.
4.1 Solutions of Quadratic Equations
Although Euclid, in keeping with the philosophy of his time, rejected all numbers
he still indirectly considered quadratic equations. His version of the solution of the
quadratic equation ax + x
2
= b
2
is the geometric theorem:
If a straight line be bisected and a straight line be added to it in a straight
line, the rectangle contained by the whole (with the added straight line) and
the added straight line together with the square on the half is equal to the
square on the straight line made up of the half and the added straight line.
Essentially this is completing the square