Sum of Infinite Series: Find 1/sqrt(2)

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SUMMARY

The discussion centers on finding the sum of the infinite series that equals 1/sqrt(2). The user initially attempted the ratio test but found it unhelpful. A key insight provided is that the series in question is the power series expansion for cos(π/4), specifically represented as ∑_{n=0}^∞ (-1)^n/(2n)! (π/4)^{2n}. This aligns with the standard power series for cos(x), confirming the result of 1/sqrt(2>.

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Umar
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Hey guys! I just have a question regarding finding the sum of an infinite series. Attached is the image of the question. I've tried to use the ratio test but it doesn't give me the result I need which happens to be 1/sqrt(2). I feel like this is one of those power series questions, but I'm not quite sure how to deal with this. If anyone could help out, that would be greatly appreciated!

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Umar said:
Hey guys! I just have a question regarding finding the sum of an infinite series. Attached is the image of the question. I've tried to use the ratio test but it doesn't give me the result I need which happens to be 1/sqrt(2). I feel like this is one of those power series questions, but I'm not quite sure how to deal with this. If anyone could help out, that would be greatly appreciated!
This is just the power series expansion for $\cos\frac\pi4.$
 
This is \sum_{n=0}^\infty \frac{(-1)^n}{(2n)!}\left(\frac{\pi}{4}\right)^{2n}

Compare that to the power series for cos(x), \sum_{n=0}^\infty \frac{(-1)^n}{(2n)!}x^{2n} to get Opalg's result.
 
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