Question

how do i do part (iii). the answer is 2 but i got the answer based on guessing. how do i do this w/o guessing? thank you.

Answer 1

(i) 2² · 3 · 7

(ii) 3024

(iii) k = 16

(iv) 12 cm

Explanation:

Part (i)

The first few prime numbers are: 2, 3, 5, 7, 11, 13, 17, ...

To find which prime numbers multiply together to make 84, start by dividing 84 by the first prime number, 2:

⇒ 84 ÷ 2 = 42

As 42 is not a prime number, we need to divide again:

⇒ 42 ÷ 2 = 21

As 21 is not a prime number, we need to divide again.

21 is not divisible by 2, so let's try dividing by the next prime number, 3:

⇒ 21 ÷ 3 = 7

As 7 is a prime number, we can stop.

Therefore, 84 is the product of:

⇒ 84 = 2 · 2 · 3 · 7

As 2 appears two times, we can write this using exponents:

⇒ 84 = 2² · 3 · 7

Part (ii)

Lowest Common Multiple (LCM): The lowest multiple shared by two or more numbers.

Prime Factors Method of finding LCM

Step 1

Find the prime factorization of each number.

From part (i): 84 = 2² · 3 · 7

Given in part (ii): 432 = 2⁴ · 3³

Step 2

Write each number as a product of primes, matching primes vertically where possible:

Step 3

Bring down the primes in each column then multiply the factors to get the LCM:

⇒ LCM = 2 · 2 · 2 · 2 · 3 · 3 · 3 · 7

⇒ LCM = 2⁴ · 3³ · 7

⇒ LCM = 3024

Part (iii)

From part (ii) we know that the prime factorization of 432 is:

432 = 2⁴ · 3³

A perfect cube is a number multiplied by itself three times.

One of the factors of 432 is a perfect cube → 3³.

Therefore,

`\begin{aligned}432 & = 2^4 \cdot 3^3\\\\\implies (432)/(2^4) & = 3^3\\\\(432)/(16) & = 3^3\end{aligned}`

So we need to divide 432 by 2⁴ to get the perfect cube 3³.

Therefore,

⇒ k = 2⁴

⇒ k = 2 · 2 · 2 · 2

⇒ k = 16

Part (iv)

To find the largest possible length of the side of the square, we need to find the greatest common factor of 432 and 84.

Prime Factors Method of finding GCF

Step 1

Find the prime factorization of each number.

From part (i): 84 = 2² · 3 · 7

Given in part (ii): 432 = 2⁴ · 3³

Step 2

Write each number as a product of primes, matching primes vertically where possible:

Step 3

Bring down the primes in the columns where both numbers have the factor, then multiply the factors to get the GCF:

⇒ GCF = 2 · 2 · 3

⇒ GCF = 12

Therefore, the largest possible length of the side of the square is 12 cm.