Among all of the rectangles whose perimeters are 100 ft, find the dimensions of the one with maximum area

I dont remember the formula but if you play with number here is what happens. divide 100 by 4, 25. 25 x 25 is 625. now lets change it to a different size just slightly different. 20 x 30 is 600. now again with a bigger change, 10 x 40, 400. it seems that the closer to even sides the bigger the area so i would go with 25 x 25.