Number systems

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# 3.5 Further exercises

## Exercise 51

Evaluate the following sums and products in modular arithmetic.

• (a)  21 +26 15,     21 ×26 15.

• (b)  19 +33 14,     19 ×33 14.

### Solution

• (a)  21 +26 15 = 10,     21 ×26 15 = 3.

• (b)  19 +33 14 = 0,     19 ×33 14 = 2.

## Exercise 52

Use Euclid's Algorithm to find:

• (a)  the multiplicative inverse of 8 in 21;

• (b)  the multiplicative inverse of 19 in 33.

### Solution

• (a)

• Hence

• Hence 8 × 8 = 3 × 21 + 1, so

• so the multiplicative inverse of 8 in 21 is 8.

• (b)

• Hence

• Hence

• so

• so the multiplicative inverse of 19 in 33 is 7.

## Exercise 53

Construct the multiplication table for 11, and hence find the multiplicative inverse of every non-zero element in 11.

### Solution

Hence we have the following multiplicative inverses in 11.

## Exercise 54

Use the solution to Exercise 52 to solve the following equations.

• (a)  8 ×21x = 13

• (b)  19 ×33x = 15

### Solution

• (a)  We have 8 ×21x = 13. Multiplying by 8, which is the multiplicative inverse of 8 mod 21 (see the solution to Exercise 52(a)), we have

• so

• (b)  We have 19 ×33x = 15. Multiplying by 7, which is the multiplicative inverse of 19 mod 33 (see the solution to Exercise 52(b)), we have

• so

## Exercise 55

Find all the solutions of the following equations.

• (a)  In 8: 3 ×8x = 7,      4 ×8x = 7,    4 ×8x = 4.

• (b)  In 15: 3 ×15x = 6,    4 ×15x = 3,    5 ×15x = 2.

### Solution

• (a)  Because 3 and 8 are coprime, the equation 3 ×8x = 7 has a unique solution. The solution, x = 5, can be found in various ways: for example, by calculating x = 3−1 ×8 7, or by testing possible values for x, or by spotting that 7 ≡ 15 (mod 8) and using the fact that the congruence 3 × 5 ≡ 15 (mod 8) implies 3 × 5 ≡ 7 (mod 8) giving 3 ×8 5 = 7.

• The equation 4 ×8x = 7 has no solutions because 4 and 8 have common factor 4 but 7 does not.

• One solution of the equation 4 ×8x = 4 is x = 1. Also n/d = 8/4 = 2, so the other solutions are x = 1 + 2 = 3, x = 1 + 4 = 5 and x = 1 + 6 = 7.

• (b)  One solution of the equation 3 ×15x = 6 is x = 2. Also n/d = 15/3 = 5, so the other solutions are x = 2 + 5 = 7 and x = 2 + 10 = 12.

• Because 4 and 15 are coprime, the equation 4 ×15x = 3 has a unique solution. The solution, x = 12, can be found in various ways: for example, by calculating x = 4−1 ×15 3, or by testing possible values for x, or by spotting that 3 ≡ −12 (mod 15) and using the fact that the congruence 4 × (−3) ≡ −12 (mod 15) implies 4 × 12 ≡ 3 (mod 15) giving 4 ×15 12 = 3.

• The equation 5 ×15x = 2 has no solutions because 5 and 15 have common factor 5 but 2 does not.

## Exercise 56

• (a)  Show that the equation x ×12x = 7 has no solutions.

• (b)  Find all the solutions of x ×12x = 4.

### Solution

We find all the values of x ×12x.

• (a)  Hence there is no integer x 12 such that x ×12x = 7.

• (b)  The solutions of x ×12x = 4 are x = 2, 4, 8, 10.

M208_6

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