Operations on Real Numbers

Real numbers are those numbers that are a combination of rational numbers and irrational numbers in the number system of maths. All the arithmetic operations like addition, subtraction, multiplication, etc. can be performed on these numbers. Besides imaginary numbers are not real numbers. Imaginary numbers are used for defining complex numbers. To get real numbers, first, we have to understand rational numbers and irrational numbers. Rational numbers are those numbers that can be written as p/q where p is numerator and q is dominator and p and q are integers. For example, 5 can be written as 5/1so it is a rational number and irrational numbers are those numbers that cannot be written in form of p/q. 

For example √3 is irrational numbers, it can be written as 1.73205081 and continuous to infinity, and it cannot be written in form of fraction and is non-terminating form and non-recurring decimals. And if combine rational numbers and irrational numbers it became real numbers.

Example: 12, -8, 5.60, 5/1, π(3.14) etc.

Real numbers can be positive and negative, and it is denoted by R. All the decimals, natural numbers and fraction come under this category.

Operations on Real Numbers

The four basics mathematical operations addition, division, multiplication, and subtraction. Now we will understand these operations on both rational and irrational numbers.

Operation on Two Rational Numbers

When we perform arithmetic operations on two rational numbers like addition, subtraction, division, and multiplication then the result will be rational numbers.

Example:

0.25 + 0.25 = 0.50 can be written as 50/100 which is form of p/q.

0.20 – 0.10 = 0.10 can be written as 10/100 which is form of p/q.

0.4 multiplied by 184 is 73.6 and can be written as 736/10.

0.252 divided by 0.4 is 0.63 and can be written as 63/100.

Operations on Two Irrational Numbers

When we perform arithmetic operations like addition, subtraction, multiplication, or division on two irrational numbers then the result can be rational numbers or irrational numbers.

Example:

√2 + √3 = 3.14 can be written as 314/100 which is rational number.

√3 – √3 = 0 or 5√4 – 4√3 = 3.07 which can be written as 307/100 which is rational number.

When √5 is multiplied to √5, we get 5 which is rational number or when √3 is multiplied to √5, we get √15 which is irrational number. When √8 is divided by √8 we get 8 which is rational number or if √5 is divided by √3 then we get (√5)/(√3) which is irrational number.

Operation on a rational number and an irrational number

Addition

When we add an irrational number and a rational number then the result will be an irrational number. When 3 is added to 2√5 then the result will be an irrational number.

Subtraction

When we perform subtraction on an irrational number and a rational number then the result will be an irrational number. When 5√6 is subtracted to 3 then the result will be an irrational number.

Multiplication

When we perform this operation the result can be irrational or rational. When 3 is multiplied to √5 then the result will be 3√5 which is an irrational number and if √12 is multiplied to √3 then the result will be √36 and it can be written as 6 which is a rational number.

Division

A rational number is divided by an irrational number or vice versa then the result will be always an irrational number. When 4 is divided by √2 then results will be 4√2 which is an irrational number.

Properties of Real Numbers

We have four properties which are commutative property, associative property, distributive property, and identity property. Consider a, b and c are three real numbers. Then these properties can be described as

Commutative Property

If a and b are the numbers, then a + b = b + a for addition and a × b = b × a for multiplication.

Addition: 

a + b = b + a;

5 + 6 = 6 + 5

Multiplication: 

a × b = b × a; 

4 × 2 = 2 × 4

Associative Property

If a, b and c are the real numbers then the form will be

a + (b + c) = (a + b) = c for addition and (a.b)c = a(b.c) for multiplication

Addition: 

a + (b + c) = (a + b) = c ; 

5 + (3 + 2) = (5 + 3) + 2

Multiplication: 

(a.b)c = a(b.c) ; 

(4×2)×6 = 4×(2×6)

Distributive Property

If a, b and c are the real numbers then the final form will be

a (b + c) = ab + ac and (a + b) c = ac + ab

5 (2+3)=5×2+5×3 the answer will be 25 for both left and right term.

Identity Property

Addition: a + 0 = 0 (0 is the additive identity)

Multiplication: a×1=1×a=1 (1 is multiplicative identity)

Real Numbers

Rational numbers: 4/5, 0.82

Integer numbers: {… – 3, -2, -1,0,1,2,3…}

Whole numbers: {0,1,2,3…}

Natural numbers: {1,2,3…}

Irrational numbers: √2, π, 0.102012…

Sample Problems

Question 1. Show that 3√7 is an irrational number.

Solution:

Let us assume, to the contrary, that 7√7 is rational.

That is, we can find coprime a and b (b ≠ 0) such that 7√7 = ab

Rearranging, we get √7 = ab/7

Since 7, a and b are integers, ab/7 is rational, and so √7 is rational.

But this contradicts the fact that √7 is irrational.

So, we conclude that 7√7 is irrational.

Question 2. Explain why (17 × 5 × 13 × 3 × 7 + 7 × 13) is a composite number?

Solution:

17 × 5 × 13 × 3 × 7 + 7 × 13 …(i)

= 7 × 13 × (17 × 5 × 3 + 1)

= 7 × 13 × (255 + 1)

= 7 × 13 × 256

Number (i) is divisible by 2, 11 and 256, it has more than 2 prime factors.

Therefore, (17 × 5 × 13 × 3 × 7 + 7 × 13) is a composite number.

Question 3. Prove that 3 + 2√3 is an irrational number.

Solution:

Let us assume to the contrary, that 3 + 2√3 is rational.

So that we can find integers a and b (b ≠ 0).

Such that 3 + 2√3 = ab, where a and b are coprime.

Rearranging the equations, we get since a and b are integers, we get a²b−32 is rational and so √3 is rational.

But this contradicts the fact that √3 is irrational.

So we conclude that 3 + 2√3 is irrational.

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