Let one rectangle A has dimensions 12 cm by 8 cm
Area of the rectangle A = l *w
Area of the rectangle A = 12 * 8 = 96 sq.cm
If we shade 1/2 of area of rectangle A Then area of shaded region = 96/2 = 48 sq.cm
Let one rectangle B has dimensions 8 cm by 6 cm
Area of the rectangle A = l *w
Area of the rectangle A = 8 * 6 = 48 sq.cm
If we shade 1/2 of area of rectangle A Then area of shaded region = 48/2 = 24 sq.cm
So if the dimensions of two rectangles are different then the half of each rectangle will be different.