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Poetry is as exact a science as geometry.
Robert Recorde's Whetstone of Witte
This is the title page of the Whetstone of Witte (1557) by Robert Recorde (1510-1558). Recorde explains in this poem the reason for the name of what is essentially an algebra text, one of the earliest in England.
On this page (Sig. S, f. i v & f. 2 r), Recorde explains the notation for a unknown and its various powers. Note that the owner of this particular copy wrote notes to help him understand the various names and abbreviations for the powers.
Recorde explains subtraction of polynomials by use of a poem (Sig. X, f. ii r).
On this page (Sig. Ff, f. i r), we see Recorde introducing, for the first time, the "equal" sign. He explains that he picked two parallel lines to represent this concept "because no two things can be more equal." He then gives various examples of the use of the equal sign in algebraic equations.
On these pages (Sig. Ii, f. iv r & v and Sig. Kk, f. i r) is Recorde's attempt to design a real problem whose solution requires a quadratic equation. This problem is entitled a "question of jorneying" and requires knowledge of the formula for the sum of an arithmetic progression.