10001 2 -9999 ​2

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10001²-9999²

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This answer shouldn't be answered, but the solution is 40,000.

Hope that was helpful. ^^

Straight

You should be able to recognise this as the difference of two squares:  a² - b² = (a+b)*(a-b)

Here, a = 10001 and b = 9999, so this is actually easy to calculate in your head.