The ratio of the areas is the square of the ratio of the sides. If a:b is the ratio of the sides then a^2:b^2 is the ratio of the areas. The reason is this. If the ratio of the sides is R (R:1) then to get an area we have to multiply in some way two sides: Ra*Rb=R^2ab. So the area of one figure is represented by ab and the other by R^2ab, so you can see that R^2ab/ab=R^2 (R^2:1 is the ratio).