Do you understand?

Is $ 2 + 2^2 + 2^3 + 2^4 + ... + 2^{10}$ divisible by $3$?

It seems undetermined, because there were no threes, but let's try to pair it, extract it to find the threes.

$A = (2 + 2^2) + (2^3 + 2^4)+...+ (2^9 + 2^{10})$

$A = 2(1+2) + 2^3(1+2) + ... + 2^9(1+2)$

$A = 2 \times 3 + 2^3 \times 3 + ... + 2^9 \times 3$

$A = (2 +2^3 + ... + 2^9) \times 3$

Look! The series has x3! So A is divisible by 3.

#Calculus

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Comments

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Yes it is divisible!!!!!!!!!!!!!

Sudhir Aripirala - 6 years, 1 month ago

An easier way to prove that it is divisible by $3$ is by using modular arithmetic. Let the given expression be $Z$ .

$Z=\sum_{i=1}^{10} 2^i \equiv \sum_{i=1}^{10} (-1)^i\pmod{3}\\ \implies Z\equiv \left(\sum_{i=1}^5 (-1)^{2i}\right)+\left(\sum_{i=1}^{5} (-1)^{2i-1}\right)\pmod{3}\\ \implies Z\equiv \left(\sum_{i=1}^5 (1)\right)+\left(\sum_{i=1}^5 (-1)\right)\equiv \sum_{i=1}^5 (1-1)\pmod{3}\\ \implies Z\equiv \sum_{i=1}^5 (0)\pmod{3}\\ \implies Z\equiv 0\pmod{3}\\ \implies 3\mid Z$

where $a\mid b$ denotes that $a$ divides $b$ , or in other words, $b$ is divisible by $a$ .

Prasun Biswas - 6 years ago

I'm an 7th grade student, so I just use original numbers :)) Sorry, can't understand.

Adam Phúc Nguyễn - 6 years ago

It's nothing much complicated. You can review the modular arithmetic wikis or ask me which part of it you can't understand?! I'd be more than glad to help you out. :)

Prasun Biswas - 6 years ago

@Prasun Biswas – What is the i?

Adam Phúc Nguyễn - 6 years ago

@Adam Phúc Nguyễn – $i$ is a counter variable in the summation. Suppose, you have the sum $S=2^1+2^2+2^3+\ldots+2^6$ . You can write it as,

$S=\sum_{i=1}^6 2^i$

The initial and final values of $i$ are given in lower and upper sides of the "sigma" symbol and the value of $i$ gets incremented in each summation step.

Prasun Biswas - 6 years 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}`	$\boxed{123}$