I'm looking for the best function to apply the product of the last two elements of sublist elements to each element:  

### Example:  

    (*Input:*)
     {{x1, y1, z1}, {x2, y2, z2}, ...}  

    (*Desired output:*)
>     {{x1, y1, z1, y1 z1}, {x2, y2, z2, y2 z2}, ...}  

I know I could just use a `Do` loop with an index `k` and do it element by element with `AppendTo`, but I guess there is a faster method.