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 #1
avatar+26400 
+3

What is the remainder when the sum

\(1^2+2^2+3^2+...+2016^2 + 2017^2 + 2018^2 +2019^2 +2020^2\) 

is divided by 17?

 

\(\begin{array}{|rcll|} \hline 1^2 \pmod {17} &=& 1 \\ 2^2 \pmod {17} &=& 4 \\ 3^2 \pmod {17} &=& 9 \\ 4^2 \pmod {17} &=& 16 \\ 5^2 \pmod {17} &=& 8 \\ 6^2 \pmod {17} &=& 2 \\ 7^2 \pmod {17} &=& 15 \\ 8^2 \pmod {17} &=& 13 \\ 9^2 \pmod {17} &=& 13 \\ 10^2 \pmod {17} &=& 15 \\ 11^2 \pmod {17} &=& 2 \\ 12^2 \pmod {17} &=& 8 \\ 13^2 \pmod {17} &=& 16 \\ 14^2 \pmod {17} &=& 9 \\ 15^2 \pmod {17} &=& 4 \\ 16^2 \pmod {17} &=& 1 \\ 17^2 \pmod {17} &=& 0 \\ \hline \end{array}\)

cycle mod 17: \((1, 4, 9, 16, 8, 2, 15, 13, 13, 15, 2, 8, 16, 9, 4, 1, 0)\)

 

circle mod 17

\(\begin{array}{|rcll|} \hline && 1+ 4+ 9+ 16+ 8+ 2+ 15+ 13+ 13+ 15+ 2+ 8+ 16+ 9+ 4+ 1+ 0\pmod {17} \\ &=& 136 \pmod {17} \\ &=& \mathbf{0 \pmod {17}} \\ \hline \end{array}\)

 

2020/17 = 118 cycles remainder 14

 

remainder 14, this is the sum in the cycle from 1 to 14  mod 17

\(\begin{array}{|rcll|} \hline && 1+ 4+ 9+ 16+ 8+ 2+ 15+ 13+ 13+ 15+ 2+ 8+ 16+ 9\pmod {17} \\ &=& 131 \pmod {17} \\ &=&\mathbf{ 12 \pmod {17}} \\ \hline \end{array}\)

 

\( \begin{array}{|rcll|} \hline && 1^2+2^2+3^2+...+2016^2 + 2017^2 + 2018^2 +2019^2 +2020^2\pmod {17} \\ &=& \underbrace{0*118 \pmod {17}}_{118 \text{ cycles}} + \underbrace{\mathbf{ 12 \pmod {17}}}_{\text{remainder 14}} \\ &=& \mathbf{ 12 \pmod {17}} \\ \hline \end{array}\)

 

The remainder when the sum \(1^2+2^2+3^2+...+2016^2 + 2017^2 + 2018^2 +2019^2 +2020^2 \) is divided by 17 is \(\mathbf{12}\)

 

laugh

May 27, 2022
 #2
avatar+118723 
+1

z^2 = 2i + 2.

 

let Z = a+bi

 

Z^2= a^2-b^2+2abi

 

\(a^2-b^2=2 \qquad (1)\\2ab=2\qquad (2)\\ ab=1\\ b=1/a\\ a^2-\frac{1}{a^2}=2\\ a^4-2a^2-1=0\\ let\;\;m=a^2\\ m^2-2m-1=0\\ ...\\ m=1+\sqrt2\qquad or \qquad m=1-\sqrt2\\ \text{but m is positive so}\\ m=1+\sqrt2\\ a=\pm\sqrt{1+\sqrt2}\\ b=\pm\frac{1}{\sqrt{1+\sqrt2}} \)

 

\(a=\pm\sqrt{1+\sqrt2}\\ b=\pm\frac{1}{\sqrt{1+\sqrt2}}\\ b=\pm\frac{1}{\sqrt{1+\sqrt2}}\frac{\sqrt{1+\sqrt2}}{\sqrt{1+\sqrt2}}\\ b=\pm\frac{\sqrt{1+\sqrt2}}{1+\sqrt2}\\ b=\pm\frac{\sqrt{1+\sqrt2}}{1+\sqrt2}\times\frac{1-\sqrt2}{1-\sqrt2}\\ b=\pm\frac{\sqrt{1+\sqrt2}}{1-2}\times\frac{\sqrt{1-\sqrt2}\sqrt{1-\sqrt2}}{1}\\ b=\pm\frac{\sqrt{1-2}}{1-2}\times\frac{\sqrt{1-\sqrt2}}{1}\\ b=\pm\frac{i}{-1}\times \frac{i\sqrt{\sqrt2-1}}{1}\\ b=\pm \sqrt{\sqrt2-1}\\ \)

You seriously need to check that

 

PLUS I expect it should ahave been done by letting     \(Z=Ae^{i\theta}\)

 

 

 

LaTex

a^2-b^2=2 \qquad (1)\\2ab=2\qquad (2)\\
ab=1\\
b=1/a\\
a^2-\frac{1}{a^2}=2\\
a^4-2a^2-1=0\\
let\;\;m=a^2\\
m^2-2m-1=0\\
...\\
m=1+\sqrt2\qquad or \qquad m=1-\sqrt2\\
\text{but m is positive so}\\
m=1+\sqrt2\\
a=\pm\sqrt{1+\sqrt2}\\
b=\pm\frac{1}{\sqrt{1+\sqrt2}}

 

a=\pm\sqrt{1+\sqrt2}\\
b=\pm\frac{1}{\sqrt{1+\sqrt2}}\\
b=\pm\frac{1}{\sqrt{1+\sqrt2}}\frac{\sqrt{1+\sqrt2}}{\sqrt{1+\sqrt2}}\\
b=\pm\frac{\sqrt{1+\sqrt2}}{1+\sqrt2}\\
b=\pm\frac{\sqrt{1+\sqrt2}}{1+\sqrt2}\times\frac{1-\sqrt2}{1-\sqrt2}\\
b=\pm\frac{\sqrt{1+\sqrt2}}{1-2}\times\frac{\sqrt{1-\sqrt2}\sqrt{1-\sqrt2}}{1}\\
b=\pm\frac{\sqrt{1-2}}{1-2}\times\frac{\sqrt{1-\sqrt2}}{1}\\
b=\pm\frac{i}{-1}\times \frac{i\sqrt{\sqrt2-1}}{1}\\
b=\pm \sqrt{\sqrt2-1}\\
 

May 27, 2022
 #1
avatar+118723 
+1
May 27, 2022
May 26, 2022

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