Like I said before, the loop is irrelevant, since it only depends on the final value of i (which will be the same value as years).
Since all your example input (except perhaps forest_rate - which you still haven't stated what a relevant value would be), you have the following
Code:
area_forested = (uncut_area + (forest_rate * uncut_area)* years );
Now, uncut_area and years are integer values (100 and 20 say).
The only way you'll get a fraction is if forest_rate * uncut_area has a fractional component (printing this intermediate result might help).
But then you multiply this by years, so even if you have a fraction ending in say 0.2, you will lose that when you multiply by years.
Try
Code:
for (int i=1; i < years+1 ; ++i) {
double temp = forest_rate * uncut_area;
area_forested = (uncut_area + temp * i);
cout << i << " " << temp << endl;
}