1989 AHSME Problems/Problem 29

Problem

What is the value of the sum $S=\sum_{k=0}^{49}(-1)^k\binom{99}{2k}=\binom{99}{0}-\binom{99}{2}+\binom{99}{4}-\cdots -\binom{99}{98}?$

$\textrm{(A)}\ -2^{50}\qquad\textrm{(B)}\ -2^{49}\qquad\textrm{(C)}\ 0\qquad\textrm{(D)}\ 2^{49}\qquad\textrm{(E)}\ 2^{50}$

Solution

By the Binomial Theorem, $(1+i)^{99}=\sum_{n=0}^{99}\binom{99}{j}i^n =$ $\binom{99}{0}i^0+\binom{99}{1}i^1+\binom{99}{2}i^2+\binom{99}{3}i^3+\binom{99}{4}i^4+\cdots +\binom{99}{98}i^{98}$ .

Using the fact that $i^1=i$ , $i^2=-1$ , $i^3=-i$ , $i^4=1$ , and $i^{n+4}=i^n$ , the sum becomes:

$(1+i)^{99}=\binom{99}{0}+\binom{99}{1}i-\binom{99}{2}-\binom{99}{3}i+\binom{99}{4}+\cdots -\binom{99}{98}$ .

So, $Re[(1+i)^{99}]=\binom{99}{0}-\binom{99}{2}+\binom{99}{4}-\cdots -\binom{99}{98} = S$ .

Using De Moivre's Theorem, $(1+i)^{99}=[\sqrt{2}\mathrm{cis}(45^\circ)]^{99}=\sqrt{2^{99}}\cdot \mathrm{cis}(99\cdot45^\circ)=2^{49}\sqrt{2}\cdot \mathrm{cis}(135^\circ) = -2^{49}+2^{49}i$ .

And finally, $S=Re[-2^{49}+2^{49}i] = -2^{49}$ .

$\fbox{B}$

1989 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 28	Followed by Problem 30
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1989 AHSME Problems/Problem 29

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