How can you determine if two ratios form a proportion?

Answer 1

There are two equivalent ways

(I) Are the two ratios equivalent fractions?

If the ratios are a/b and c/d then can you find a number x such that

a = c*x and b = d*x.

so that a/b = c*x/d*x = c/d

This requires x = a/c = b/d

(II) Cross-multiply and equate.

If a/b = c/d then cross-multiplication gives ad = bc.

Also, if ad = bc then a/b = c/d (divide both sides by bd).