root of which added to or subtracted from 5 gives parts the product of which is 40. These will be 5 +√ −15 and 5−√−15. Putting aside the mental tortures involved, multiply 5 + √ −15 and 5 − √−15making 25 − (−15) which is +15. Hence this product is 40. 7. Rafael Bombelli authored l’Algebra (1572, and 1579), a set of three books. Bombelli introduces a notation for √ −1, and calls it “piu´ di meno”. The discussion of cubics in l’Algebra follows Cardano, but now the casus irreducibilis is fully discussed. Bombelli considered the equation
x3 = 15x + 4
for which the Cardan formula gives
x =
3 q2 +√−121 + 3 q2−√−121 Bombelli observes that the cubic has x = 4 as a solution, and then proceeds to explain the expression given by the Cardan formula as another expression for x = 4 as follows. He sets 3 q2 +√−121 = a + bi from which he deduces 3 q2−√−121 = a− bi and obtains, after algebraic manipulations, a = 2 and b = 1. Thus x = a + bi + a− bi = 2a = 4 After doing this, Bombelli commented:
“ At first, the thing seemed to me to be based more on sophism than on truth, but I searched until I found the proof.”
8. Ren´e Descartes (1596-1650) was a philosopher whose work, La G´eom´etrie, includes his application of algebra to geometry from which we now have Cartesian geometry. Descartes was pressed by his friends to publish his ideas, and he wrote a treatise on science under the title “Discours de la m´ethod pour bien conduire sa raison et chercher la v´erit´e dans les sciences”. Three appendices to this work were La Dioptrique, Les M´et´eores, and La G´eom´etrie. The treatise was published at Leiden in 1637. Descartes associated imaginary numbers with geometric impossibility. This can be seen from the geometric construction he used to solve the equation z2 = az −b2, with a and b2 both positive. According to [1],Descartes coined the term imaginary:
“For any equation one can imagine as many roots [as its degree would suggest], but in many cases no quantity exists which corresponds to what one imagines.”
9. John Wallis (1616-1703) notes in his Algebra that negative numbers, so long viewed with suspicion by mathematicians, had a perfectly good physical explanation, based on a line with a zero mark, and positive numbers being numbers at a distance from the zero point to the right, where negative numbers are a distance to the left of zero. Also, he made some progress at giving a geometric interpretation to √