# Solution:

We are given two equations:

1) x/y = c/d

2) d/c = b/a

Let’s now test the expressions in the Roman numerals to determine which of them are true.

I. y/x = b/a

If we reciprocate the fractions on both sides of the first equation we have: y/x = d/c.

Since the first equation can be expressed as y/x = d/c and the second equation is d/c = b/a, then we see that  y/x = b/a.

Thus, the equation in I must be true.

II. x/a = y/b

From Roman numeral I, we know that y/x = b/a.

If we multiply both sides by x, we have: y = (bx)/a. Now divide both sides by b, yielding: y/b = x/a.

Thus, the equation in II must be true.

III. y/a = x/b

From Roman numeral II, we know that x/a = y/b or y/b = x/a. However, this doesn’t mean y/a = x/b. For example, let y = 6, b = 3, x = 4 and a = 2. We see that y/b = x/a (6/3 = 4/2), but y/a ≠ x/b (6/2 ≠ 4/3).

Thus, the equation in III is not true.