These results do support the law of definite proportions. We know that A combines with B in a ratio of 9:5 by mass. If we divide 9 by 5 we get 1.8, so this ratio reduces to 1.8:1. This corresponds exactly with the second observation that 1.8 g of A reacts with 1.0 g of B, this giving a mass ratio of 1.8:1. This is the exact same mass ratio as before. The law of definite proportions states that a compound always exists as set proportions of each element to each other by mass. Since A and B combine in a constant mass ratio of 1.8:1, these observations uphold the law of definite proportions.
This Law of Definite Proportion is also known as Law of Constant Composition, This law states that the relative amount of each element in a specific compound is always the same or it can also be defined as a compound always contain the same proportion by mass of its constituent element.
From the question, element A and B combines in the ratio of 9:5 which means 9 grams of A react with 5 grams of B. If 1 gram of B is reacting, then 1.8 grams of A is required which is in the ratio of 9:5. Thus defines the Law of Definite Propotion.