Recognising prime numbers
Decide whether a whole number is prime.
- A prime number has exactly two positive factors: 1 and itself.
- The number 1 is not prime. The only even prime is 2.
- Test prime divisors up to the square root of the number.
Example 1
Is this a prime number? Explain your answer.
None of 2, 3, 5 divides 29 exactly.
Example 2
Is this a prime number? Explain your answer.
49 has a factor other than 1 and itself.
Remember
Test prime divisors up to the square root of the number.
Your turn
Is this a prime number? Explain your answer.
Check answer
None of 2, 3, 5, 7 divides 113 exactly.
Is this a prime number? Explain your answer.
Check answer
128 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
Check answer
78 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
Check answer
86 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
Check answer
98 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
None of 2, 3, 5, 7 divides 113 exactly.
Is this a prime number? Explain your answer.
128 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
78 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
86 has a factor other than 1 and itself.
Is this a prime number? Explain your answer.
98 has a factor other than 1 and itself.