CROSS PRODUCT PROPORTION
Suppose, w, x, y, and z are the 1st, 2nd, 3rd, and 4th terms of the proportion respectively then the equation would be w : x = y : z . The 1st & 4th terms are called EXTREMES and the 2nd & 3rd terms are called MEANS.
The term z is considered as a 4th and also called the Fourth Proportional to w, x, y.
1st Term 3rd Term w y
So, ------------ = ------------ ; -------- = --------
2nd Term 4th Term x z
If we multiply both the 1st Term X 4th Term = 3rd Term X 2nd Term ; so, wz = yx
The above process is called the cross product rule.
If four numbers w, x, y, & z are in proportion of which any three is known, we can know the remaining unknown number by using the fact that wz = yx .
If four numbers are in proportion then
We can conclude that, the product of the EXTREMES = the product of the MEANS
The above rule helps us to find any term of proportion if any three are known to us.