It is in the work of the French mathematician François Viète that the first consistent, coherent, and systematic conception of an algebraic equation in the modern sense appeared. A main innovation of Viète’s In artem analyticam isagoge (1591; “Introduction to the Analytic Art”) was its use of well-chosen symbols of one kind (vowels) for unknowns and of another kind (consonants) for known quantities. This allowed not only flexibility and generality in solving linear and quadratic equations but also something absent from all his predecessors’ work, namely, a clear analysis of the relationship between the forms of the solutions and the values of the coefficients of the original equation. Viète saw his contribution as developing a “systematic way of thinking” leading to general solutions, rather than just a “bag of tricks” to solve specific problems.
By combining existing usage with his own innovations, Viète was able to formulate equations clearly and to provide rules for transposing factors from one side of an equation to the other in order to find solutions. An example of an equation would be:
A cubus + C plano in A aequatus D solido
(meaning: x3 + cx = d).
Note that each of the terms involved was one-dimensional, that is, after canceling powers, the remaining terms on each side of the equation are to the first power. Thus, on the left-hand side, the two-dimensional magnitude Z plano (a square) was divided by the one-dimensional variable G, leaving one dimension. On the right-hand side, a sum of two three-dimensional magnitudes (a third power) was divided by a product of two one-dimensional variables (which make a square), leaving one dimension. Thus, Viète did not break the important Greek tradition whereby the terms equated must always be of the same dimension. Nevertheless, for the first time it became possible, in the framework of an equation, to multiply or divide both sides by a certain magnitude. The result was a new equation, homogeneous in itself yet not homogeneous with the original one.
Viète showed how to transform given equations into others, already known. For example, in modern notation, he could transform x3 + ax2 = b2x into x2 + ax = b2. He thus reduced the number of cases of cubic equations from the 13 given by Cardano and Bombelli. Nevertheless, since he still did not use negative or zero coefficients, he could not reduce all the possible cases to just one.
Viète applied his methods to solve, in a general, abstract-symbolic fashion, problems similar to those in the Diophantine tradition. However, very often he also rephrased his answers in plain words—as if to reassure his contemporaries, and perhaps even himself, of the validity of his new methods.
