In a proportion a : b :: c : d, the product of the extremes (a × d) equals the product of which terms?

If a : $b=c:d$, the four numbers are in proportion. The classic check: the **product of the extremes** $(a\times d)$ equals the **product of the means** $(b\times c)$. Example: are 4 : 6 and 6 : 9 in proportion? Cross-multiply: $4\times 9=36,6\times 6=36$. Equal → yes, in proportion.