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 #1
avatar+33616 
+1
Jul 3, 2020
 #2
avatar+26367 
+2

Find the value of the sum \(\dbinom{99}{0} - \dbinom{99}{2} + \dbinom{99}{4}- \dbinom{99}{6} + \dots - \dbinom{99}{98}\)

 

 

\((1+i)^{99}=\dbinom{99}{0}+i\dbinom{99}{1}-\dbinom{99}{2}-i\dbinom{99}{3}+\dbinom{99}{4}+i\dbinom{99}{5}- \ldots -\dbinom{99}{98}-i\dbinom{99}{99} \qquad (1)\)

 

Note here that \(\binom{99}{0} - \binom{99}{2} + \binom{99}{4}- \binom{99}{6} + \dots - \binom{99}{98}\) is the real part of (1).

 

 

\(\begin{array}{|rcll|} \hline 1+i &=& \sqrt{1^2+1^2} \Bigg(\cos\Big( \arctan(\frac{1}{1} )\Big) +i\sin\Big( \arctan(\frac{1}{1} )\Big) \Bigg) \\ 1+i &=& \sqrt{2} \Big(\cos(45^\circ) +i\sin(45^\circ) \Big) \\ \hline (1+i)^{99} &=& \Bigg( \sqrt{2} \Big(\cos(45^\circ) +i\sin(45^\circ) \Big) \Bigg)^{99} \\ (1+i)^{99} &=& 2^{\frac{99}{2}} \Big(\cos(99*45^\circ) +i\sin(99*45^\circ) \Big) \\ (1+i)^{99} &=& 2^{\frac{99}{2}} \Big(\cos(4455^\circ) +i\sin(4455^\circ) \Big) \\ (1+i)^{99} &=& 2^{\frac{99}{2}} \Big(\cos(135^\circ) +i\sin(135^\circ) \Big) \\ (1+i)^{99} &=& 2^{\frac{99}{2}} \Big(-\dfrac{\sqrt{2}}{2} +i\dfrac{\sqrt{2}}{2} \Big) \\ (1+i)^{99} &=& 2^{\frac{99}{2}} \Big(-2^{\frac{1}{2}-1} +i\dfrac{\sqrt{2}}{2} \Big) \\ (1+i)^{99} &=& 2^{\frac{99}{2}} \Big(-2^{-\frac{1}{2}} +i\dfrac{\sqrt{2}}{2} \Big) \\ (1+i)^{99} &=& 2^{\frac{99}{2}}*(-2^{-\frac{1}{2}}) +i*2^{\frac{99}{2}} *\dfrac{\sqrt{2}}{2} \\ (1+i)^{99} &=& -2^{\frac{99}{2}}2^{-\frac{1}{2}} +i*2^{\frac{99}{2}} *\dfrac{\sqrt{2}}{2} \\ (1+i)^{99} &=& -2^{\frac{99}{2}-\frac{1}{2}} +i*2^{\frac{99}{2}} *\dfrac{\sqrt{2}}{2} \\ (1+i)^{99} &=& -2^{\frac{98}{2}} +i*2^{\frac{99}{2}} *\dfrac{\sqrt{2}}{2} \\ (1+i)^{99} &=& \underbrace{\color{red}-2^{49}}_{\text{the real part}} +i*2^{\frac{99}{2}} *\dfrac{\sqrt{2}}{2} \\ \hline \end{array} \)

 

\(\dbinom{99}{0} - \dbinom{99}{2} + \dbinom{99}{4}- \dbinom{99}{6} + \dots - \dbinom{99}{98} = \mathbf{-2^{49}}\)

 

laugh

Jul 3, 2020
 #1
avatar+26367 
+3

Let a < b < c < d be four consecutive positive integers such that     

- a is divisible by 5,     
- b is divisible by 7,    
- c is divisible by 9, and      
- d is divisible by 11. 

Find the minimum value of a+b+c+d. 

 

\(\begin{array}{ll} \left.\begin{array}{rcl} a & \equiv & 0 \pmod{5} \\ b=a+1 & \equiv & 0 \pmod{7} \\ c=a+2 & \equiv & 0 \pmod{9} \\ d=a+3 & \equiv & 0 \pmod{11} \end{array}\right\} \begin{array}{rcl} a & \equiv & 0 \pmod{5} \\ a & \equiv & -1 \pmod{7} \\ a & \equiv & -2 \pmod{9} \\ a & \equiv & -3 \pmod{11} \end{array} \\\\ a+b+c+d = 4a+6 \\ \end{array} \)

 

\(\begin{array}{ll} \left.\begin{array}{rcl} a & \equiv & 0 \pmod{5} \\ a & \equiv & -1 \pmod{7} \\ \end{array}\right\} \begin{array}{rcl} a & \equiv & -15 \pmod{35} \\ \end{array} \\ \left.\begin{array}{rcl} a & \equiv & -2 \pmod{9} \\ a & \equiv & -3 \pmod{11} \end{array}\right\} \begin{array}{rcl} a & \equiv & -47 \pmod{99} \\ \end{array} \end{array} \)

 

\(\begin{array}{ll} \left.\begin{array}{rcl} a & \equiv & -15 \pmod{35} \\ a & \equiv & -47 \pmod{99} \\ \end{array}\right\} \begin{array}{rcl} a & \equiv & 1735 \pmod{3465} \\ a &=& 1735 + 3465n,\ n\in \mathbb{Z} \\ \end{array} \\ \end{array} \)

 

\(\text{The minimum value of $\mathbf{a}$ is $\mathbf{1735}$} \\ \text{The minimum value of $\mathbf{a+b+c+d} = 6*1735 + 4 \mathbf{=6946}$ } \)

 

Check:

\(\begin{array}{|rcll|} \hline 1735 & \equiv & 0 \pmod{5} \\ 1736 & \equiv & 0 \pmod{7} \\ 1737 & \equiv & 0 \pmod{9} \\ 1738 & \equiv & 0 \pmod{11} \\ \hline \end{array}\)

 

laugh

Jul 3, 2020
 #1
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Jul 3, 2020

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