Proposition 18 . Given two numbers, to investigate whether it is possible to find a third proportional to them. Proposition 19 . Given three numbers, to investigate when it is possible to find a fourth proportional to them. Proposition 20 .
Prime numbers are more than any assigned multitude of prime numbers.
Proposition 21 . If as many even numbers as we please are added together, then the sum is even. Proposition 22 . If as many odd numbers as we please are added together, and their multitude is even, then the sum is even. Proposition 23 .