In the proportion a/b = c/d, which equation results from cross-multiplication?

In the proportion a/b = c/d, which equation results from cross-multiplication?

• A. ac = bd
• B. a + b = c + d
• C. ad = bc
• D. ab = cd

Answer: (C)

Cross-multiplying the ratio a/b = c/d means clearing the fractions by multiplying both sides by b d. The left side becomes a d and the right side becomes b c, giving ad = bc. This is the direct consequence of the equality of the two fractions. The other forms don’t come from this step: adding the numbers isn’t how you preserve equality of fractions, and ac = bd or ab = cd come from different setups or manipulations that aren’t the cross-multiplication of this proportion.

Cross-multiplying the ratio a/b = c/d means clearing the fractions by multiplying both sides by b d. The left side becomes a d and the right side becomes b c, giving ad = bc. This is the direct consequence of the equality of the two fractions.

The other forms don’t come from this step: adding the numbers isn’t how you preserve equality of fractions, and ac = bd or ab = cd come from different setups or manipulations that aren’t the cross-multiplication of this proportion.