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Which Is Bigger?

Age 14 to 16
Challenge Level

 

You may wish to explore the problem Which Is Cheaper? before working on this task.

Which is bigger, $n+10$, or $2n+3$?
How did you decide?


Here's how I decided: 

 

 

"I wonder what happens when $n=4$."

 

"$4+10=14$ but $2 \times 4 + 3$ is only $11$."
"So it looks like $n+10$ is bigger."

 

 

But then my friend said:

 

 

"I wonder what happens when $n=10$."
"$10+10=20$ but $2 \times 10 +3$ is $23$."
"So it looks like $2n+3$ is bigger."

 

Can you explain why we have come to different conclusions?
Is there a diagram you could draw that would help?



For the following pairs of expressions, can you work out when each expression is bigger?
 

 

$2n+7$ and $4n+11$
$2(3n+4)$ and $3(2n+4)$
$2(3n+3)$ and $3(2n+2)$
 

Here are some challenges to try:
  • Find two expressions so that one is bigger whenever $n< 5$ and the other is bigger whenever $n> 5$.
  • Find three expressions so that the first is biggest whenever $n< 0$, the second is biggest whenever $n$ is between 0 and 4, and the third is biggest whenever $n> 4$.
  • Find three expressions so that the first is biggest whenever $n< 3$, the second is biggest when $n> 3$, and the third is never the biggest.
  • Find three expressions so that one of them is the biggest regardless of the value of $n.$


You may also be interested in the other problems in our Many ways to see Feature.

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