"On symbolic use of equalities and proportions. Chapter II. The analytical method accepts as proven the most famous [ as known from Euclid ] symbolic use of equalities and proportions that are found in items such as: 1. The whole is equal to the sum of its parts. 2. Quantities being equal to the same quantity have equality between themselves. [a = c & b = c => a = b] 3. If equal quantities are added to equal quantities the resulting sums are equal. 4. If equals are subtracted from equal quantities the remains are equal. 5. If equal equal amounts are multiplied by equal amounts the products are equal. 6. If equal amounts are divided by equal amounts, the quotients are equal. 7. If the quantities are in direct proportion so also are they are in inverse and alternate proportion. [a:b::c:d=>b:a::d:c & a:c::b:d] 8. If the quantities in the same proportion are added likewise to amounts in the same proportion, the sums are in proportion. [a:b::c:d => (a+c):(b+d)::c:d] 9.If the quantities in the same proportion are subtracted likewise from amounts in the same proportion, the differences are in proportion. [a:b::c:d => (a-c):(b-d)::c:d] 10. If proportional quantities are multiplied by proportional quantities the products are in proportion. [a:b::c:d & e:f::g:h => ae:bf::cg:dh] 11. If proportional quantities are divided by proportional quantities the quotients are in proportion. [a:b::c:d & e:f::g:h => a/e:b/f::c/g:d/h] 12. A common multiplier or divisor does not change an equality nor a proportion. [a:b::ka:kb & a:b::(a/k):(b/k)] 13. The product of different parts of the same number is equal to the product of the sum of these parts by the same number. [ka + kb = k(a+b)] 14. The result of successive multiplications or divisions of a magnitude by several others is the same regardless of the sequential order of quantities multiplied times or divided into that magnitude. But the masterful symbolic use of equalities and proportions which the analyst may apply any time is the following: 15. If we have three or four magnitudes and the product of the extremes is equal to the product means, they are in proportion.[ad=bc => a:b::c:d OR ac=b2 => a:b::b:c] And conversely 10. If we have three or four magnitudes and the first is to the second as the second or the third is to the last, the product of the extremes is equal to that of means. [a:b::c:d => ad=bc OR a:b::b:c => ac=b2] We can call a proportion the establishment of an equality [equation] and an equality [equation] the resolution of a proportion."