A Neat Trick!

A 6-year old once calculated \(1+2+3+\ldots+99+100 \) in under 10 seconds.  How did he do it?  (No, he didn't use a computer.)  If you would like a hint, click here.

Once, you see the trick, calculate the following (without a calculator). 
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1. \( 1+2+\ldots +37+38 \)
2. \( 2+4+6+\ldots +98+100 \)
3. \( 1+230+31+32+33\ldots +79+80 \)

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