Fast Modular Arithmetic

What is the fastest way to show that $102^2 \equiv 30\mod {247}$ ?

#NumberTheory

This discussion board is a place to discuss our Daily Challenges and the math and science related to those challenges. Explanations are more than just a solution — they should explain the steps and thinking strategies that you used to obtain the solution. Comments should further the discussion of math and science.

When posting on Brilliant:

Use the emojis to react to an explanation, whether you're congratulating a job well done , or just really confused .
Ask specific questions about the challenge or the steps in somebody's explanation. Well-posed questions can add a lot to the discussion, but posting "I don't understand!" doesn't help anyone.
Try to contribute something new to the discussion, whether it is an extension, generalization or other idea related to the challenge.
Stay on topic — we're all here to learn more about math and science, not to hear about your favorite get-rich-quick scheme or current world events.

Markdown	Appears as
`italics` or `_italics_`	italics
`bold` or `__bold__`	bold
- bulleted - list	bulleted list
1. numbered 2. list	numbered list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1 paragraph 2	paragraph 1 paragraph 2
`[example link](https://brilliant.org)`	example link
`> This is a quote`	This is a quote
# I indented these lines # 4 spaces, and now they show # up as a code block. print "hello world"	# I indented these lines # 4 spaces, and now they show # up as a code block. print "hello world"

Math	Appears as
Remember to wrap math in `$` ... `$` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$

Comments

I suppose one way would be to first note that $102 = 124 - 22$ , and so

$A = 102^{2} = (124 - 22)^{2} = 124^{2} - 2*22*124 + 22^{2} = 62*248 - 22*248 + 484.$

Now $248 \equiv 1 \pmod{247}$ and $484 = 2*248 - 10 \equiv -10 \pmod{247}$ , so

$A \equiv (62*1 - 22*1 - 10) \pmod{247} \equiv 30 \pmod{247}$ .

Brian Charlesworth - 5 years, 3 months ago

@Calvin Lin @Pi Han Goh @Otto Bretscher

Axas Bit - 5 years, 4 months ago

@Brian Charlesworth Any thoughts?

Axas Bit - 5 years, 4 months ago

Brian Charlesworth gave a standard answer. Here's an alternative approach:

Notice that $50^2 = 2500 = 30 + 2470 = 30 + 247(10)$ , then $102^2 - 50^2 = (102 - 50)(102 + 50) = 52 \times 152 = 247 \times (\ldots ).$

This tells us that $102^2 - 50^2 \equiv 0 \pmod {247} \Rightarrow 102^2 \equiv 50^2 \pmod{247} \Rightarrow 10^2 \equiv 30 \pmod{247}$ .

Pi Han Goh - 5 years, 3 months ago

'Fastest way' should definitely be checking if 247 divides $102^2-30$ . We find that it is true very fast.

A Former Brilliant Member - 5 years, 3 months ago

Define "fastest".

In terms of no need to think about what to do: $102^2 = 42 \times 247 + 30$ .

Calvin Lin Staff - 5 years, 3 months ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$