“ Section 39 If the first two terms are added together and the last two terms are added together, the ratio of the sum of the first addition to the first term is equivalent to the ratio of the sum of the second addition to the third term. ”

“ Section 44 From these methods it becomes clear that given two equal planes, if one places two of the edges of one plane as the extremes and two edges of the other plane as the means they will be in proportion to each other. ”

“ Section 43 If the first term is multiplied by the fourth term, it will be equal to the number returned by multiplying the second term by the third term, like so: 2:3 = 4:6 has extremes whose multiplication produce 12 as well as means whose multiplication produce 12. ”