The eight proposition.
If a number be deuided into two numbers, the superficiall number produced of the multiplication of the whole into one of the partes foure tymes, together with the square of the other parte, is equall to the square of the number composed of the whole number and the foresayd part.
Suppose that the number AB be deuided into two numbers AC and CB. Then I say that the superficiall number produced of the multiplication of the number AB in to the number CB foure tymes together with the square of the number AC, is equall to the square of the number composed of the numbers AB & CB. For vnto the num∣ber BC let the number BD be equall. Now forasmuch as the square of the
number
AD is equal to the squares of the numbers
AB and
BD, & to the superficiall number produced of the multiplication of the numbers
AB &
BD the one into the other twise (by the 4. of this booke) And the number
BD is equall to the number
BC: therefore the square of the number
AD is equall to the squares of the numbers
AB and
BC, and to the superficiall number produced of the multiplication of the numbers
AB and
BC the one into the other twise. But the squares of the numbers
AB and
BC are e∣quall vnto the superficiall number produced of the multiplication of the numbers
AB and
BC the one into the other twise, and to the square of
AC (by the former proposition) Wherfore the square of the number
AD is equall to the superficial number produced of the multiplication of the nū∣bers
AB and
BC the one into the other foure tymes, and to the square of the number
AC. But the square of the number
AD is the square of the number composed of the numbers
AB and
BC: for the number
BD is e∣qual to the number
BC. Wherfore the square of the number composed of the numbers
AB and
BC is equall to the superficiall number produced of the multiplication of the numbers
AB and
BC the one into the other foure tymes, & to the square of the num∣ber
AC. If therfore a number be deuided into two numbers, &c.
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