12. In the figure, there are two squares of different sizes. The area of the shaded part is 1.25 times the area of the blank part. When the side length of the larger square is $a$, the side length of the smaller square is
【Analysis】The area of the large square is $1+1.25=2.25=1.5^{2}$ times that of the small square, so the side length of the large square is 1.5 times that of the small square, so the side length of the small square $a \div 1.5=\frac{2}{3} a$