We will discuss here about the replacement set and solution set in set notation.

**Replacement Set:** The set, from which the values of the variable which involved in the inequation, are chosen, is known as replacement set.

**Solution Set:** A solution to an inequation is a number chosen from the replacement set which, satisfy the given inequation. The set of all solutions of an inequation is known as solution set of the inequation.

For example:

Let the given inequation be y < 6, if:

(i) The replacement set = N, the set of natural numbers;

The solution set = {1, 2, 3, 4, 5}.

(ii) The replacement set = W, the set of whole numbers;

The Solution set = {0, 2, 3, 4, 5}.

(iii) The replacement set = Z or I, the set of integers;

The solution set = {........., -4, -3, -2, -1, 0, 1, 2, 3, 4, 5}

But, if the replacement set is the set of real numbers, the solution set can only be described in set-buider form, i.e., {x : x ∈ R and y < 6}.

Solved example on replacement set and solution set in set notation:

**1.** If the replacement set is the set of whole numbers (W),
find the solution set of 4z – 2 < 2z + 10.

**Solution: **

4z – 2 < 2z + 10

⟹ 4z – 2 + 2< 2z + 10 + 2, [Adding 2 on both the sides]

⟹ 4z < 2z + 12

⟹ 4z – 2z < 2z + 12 – 2z, [Subtracting 2z from both sides]

⟹2z < 12

⟹ \(\frac{2z}{2}\) < \(\frac{12}{2}\), [Dividing both sides by 2]

⟹ z < 6

Since the replacement set = W (whole numbers)

Therefore, the solution set = {0, 1, 2, 3, 4, 5}

**2.** If the replacement set is the set of real numbers (R),
find the solution set of 3 - 2x < 9

**Solution: **

3 - 2x < 9

⟹ - 2x < 9 – 3, [by transferring 3 on the other side]

⟹ -2x < 6

⟹ \(\frac{-2x}{-2}\) > \(\frac{6}{-2}\), [Dividing both sides by -2]

⟹ x > -3

Since the replacement set = R (real numbers)

Therefore, the solution set = {x | x > -3, x ∈ R}.

**3.** If the replacement set is the set of integers, (I or Z),
between -6 and 8, find the solution set of 15 – 3d > d - 3

**Solution: **

15 – 3d > d - 3

⟹ 15 – 3d - 15 > d – 3 – 15, [Subtracting 15 from both sides]

⟹ -3d > d - 18

⟹ -3d - d> d – 18 – d, [Subtracting d from both sides]

⟹-4d > -18

⟹ \(\frac{-4d}{-4}\) < \(\frac{-18}{-4}\), [Dividing both sides by -4]

⟹ d < 4.5

Since, the replacement is the set of integers between -6 and 8

Therefore, the solution set = {-5, -4, -3, -2, -1, 0, 1, 2, 3, 4}

__From Condition of Perpendicularity of Two Straight Lines____ to HOME__

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