Question

Let R be a relation on N defined by x+2y=8.The domain of R is
A. {2,4,6}
B. {2,4,6,8}
C. {2,4,6}
D. {1,2,3,4}

Answer 1

Hint-Find out the equation for either x or y and then find out the respective values of x and y.

We have been given with a relation R on set of natural numbers defined by x+2y=8
From this, we can write y=${\displaystyle \frac{8-x}{2}}$
Since, x and y are natural numbers , the value of x and y has to be positive and also zero is excluded,
Also, from the equation , we can infer that the value of x has to be an even number , only then we will get y as a natural number
So, let’s start from the least natural even number and proceed
So, in the first case, we get
If x=2, y=${\displaystyle \frac{8-2}{2}}=3$
If x=4,$y={\displaystyle \frac{8-4}{2}}=2$
If x=6,$y={\displaystyle \frac{8-6}{2}}=1$
If x=8,$y={\displaystyle \frac{8-8}{2}}=0$ , but 0 does not belong to the set of natural numbers
So even the only possible values of x are 2,4,6
So, the relation will be R={(2,3),(4,2),(6,1)}
So, now we can write the domain of the relation R={2,4,6}
So, option C is the correct answer for this question.

Note: Please make sure to write the proper values of the domain when it is asked, do not get confused with the values of the co-domain and whenever we have problems like these make sure to write the values of the domain in the form of a set.