# Modular addition and subtraction

## Let's explore the **addition property** of modular arithmetic:

### (**A + B**) mod C = (A mod C + B mod C) mod C

### Example:

Let

**A=14, B=17, C=5**Let's verify:

**(A + B) mod C = (A mod C + B mod C) mod C****LHS**= Left Hand Side of the Equation**RHS**= Right Hand Side of the EquationLHS = (A + B) mod C

LHS = (

LHS =

LHS = (

**14 + 17**) mod 5LHS =

**31**mod 5**LHS = 1**RHS = (A mod C + B mod C) mod C

RHS = (

RHS = (

RHS = (

**14 mod 5 + 17 mod 5**) mod 5RHS = (

**4 + 2**) mod 5**RHS = 1****LHS = RHS = 1**

## Intuition Behind Modular Addition

Observe the figure below. If we want to calculate

**12+9 mod 7**we can easily go around the modular circle for a sequence of**12+9 steps**clockwise (as shown in the bottom left circle).We can take a shortcut by observing that every

**7 steps**we end up in the same position on the modular circle. These complete loops around the modular circle**don’t contribute to our final position**. We ignore these complete loops around the circle by**calculating each number mod 7**(as shown in the two upper modular circles). This will give us the number of clockwise steps, relative to 0, that contributed to each of their final positions around the modular circle.Now, we only have to go around the circle clockwise the total of the number of steps that contributed to each of numbers final position (as shown in the bottom right modular circle).

**This method applies, in general, to any two integers and any modular circle.**## Proof for Modular Addition

We will prove that

We must show that

**(A + B) mod C = (A mod C + B mod C) mod C**We must show that

**LHS=RHS**From the quotient remainder theorem we can write A and B as:

**A**= C * Q1 + R1 where 0 ≤ R1 < C and Q1 is some integer.

**A mod C = R1**

**B**= C * Q2 + R2 where 0 ≤ R2 < C and Q2 is some integer.

**B mod C = R2**

**(A + B) = C * (Q1 + Q2) + R1+R2**

**LHS**= (A + B) mod C

**LHS**= (C * (Q1 + Q2) + R1+ R2) mod C

We can eliminate the multiples of C when we take the mod C

**LHS = (R1 + R2) mod C**

**RHS**= (A mod C + B mod C) mod C

**RHS = (R1 + R2) mod C**

**LHS=RHS= (R1 + R2) mod C**

## Modular Subtraction

A very similar proof holds for modular subtraction