Because it's about the same value as the LDR's resistance. See my description about how it works.
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I spent some time writing a tutorial on how to make a Guitar Tuner using this method. You can find it on my blog: http://www.akellyirl.com/arduino-guitar-tuner/
Arduino Guitar Tuner Using DSP TechniquesView Instructable »
What you need to do is fill the rawData  array with data from the ADC. You can use a for loop for that. You should also subtract the mean of the data so that it's centred at zero.
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Add Amazon Echo voice control to you Smart DeviceView Instructable »
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Both reistors are required to make the voltage change. Read on to this section which explains what's going on: "As the light level increases the resistance drops, which makes the current increase (by Ohm's Law), which in turn, makes the voltage at A0 (Va0) increase."As both resistors are in series the same current must flow through them both. The LDR resistance drops with light, which causes the current in both resistors to increase (I=V/R), and therefore the voltage across the other (non-LDR) increases.
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